When the going gets tough …

by George J. Reclos & Marina Parha

With one of us stating Overdosing is better than Underdosing and the other advising If in doubt Underdose, debate was unavoidable. We both enjoyed each other’s “critical friend’s” remarks and felt other people may do too; we would welcome other views on this topic as it is paramount to fish keeping. So we decided to share our debate.

Editing it for presentation was a bit of a task as the rough material consisted of exchanges in a discussion format.  Discussions have a tendency to veer to all sorts of related topics; this makes for much of the pleasure but doesn’t look good in a written presentation. We have tried to shape our notes up and this resulted in some (unavoidable) repetitions of arguments under different headings to keep the consistency of each thread. We hope this will not detract from the essence.

The following topics are discussed in more or less detail below:

Isolating Sick Fish


Immunity against Pathogens

Viral Infections

“Correct” Dosing (Dosing According to Instructions)

UV Sterilisers

Sensitive Fish and Other Issues

Absorption of Medications

Resistant Strains of Pathogens


followed by our joint


Isolating sick fish

George: “Firstly, in some cases, the reasoning for isolating the fish is questionable. If the fish is heavily infected with external micro-organisms then, most probably, other fishes are infected too, or the micro organism is present in the water column. It is common sense that the micro organism was present in the water and this is how the fish was infected.” Hence treating the whole tank is a must.

Marina: In the water you get all sorts of micro-organisms. However, a balance is achieved (as with the human body) due the presence of a certain amount of “conflicting”, so to speak, micro-organisms which keep each other in check. This creates a natural balance. In addition, the immune system of the fish is supposed to fight the existing pathogens so their presence is not doing any damage. Hence the theory that fish get used to their environment but the introduction of a new fish upsets the balance as: a. it carries new or different micro - organisms and b. it hasn’t necessarily got immunity to the organisms already existing in the aquarium.

Accordingly, in a given case (b) a new arrival may be infected by organisms already present in one’s aquarium. Once infected, there is a disease outbreak. Yet, if the new fish is isolated the others will be fine as they are immune to the micro-organisms in the water.

Similarly, some species and some individuals are more sensitive to certain micro-organisms than others. This may result in some fish being affected by a disease while others are not (in the same aquarium). Isolation helps again here.

Having said that, I fully take your main point that if more than one fish is affected with the same disease the aquarist is far better off treating the whole tank (just in case) – particularly in cases of Velvet, Ich and other diseases whose spores are water-borne. I just think that it would be better if a distinction was made in the article.

George: A distinction could be made I suppose. Yet my concern is that such a distinction could only be based on an evaluation of each given situation, which would involve all aquarists, including beginners. If you have a rule which allows for interpretation by the hobbyist then you will never have a rule.  After all, what proportion of hobbyists are proficient enough to be confident that on a given occasion they should treat the whole tank or just an isolated fish?  Based on what criteria? How much is known about the immune system of fish, let alone fish collected in the Amazon and now living in a different country? Which is the effect of water conditioners or activated carbon on the immune system? Which of the macro- or micronutrients, vitamins, or trace elements which exist in the natural environment (but not in our tanks) influence the efficacy of the immune system and in what way? There are lots of issues involved. That apart, fish don’t necessarily develop immunity to pathogens, so it is better to rely on drugs. After all, the essence of the article is not to treat at all, but if you have to, then do it.

Marina: There is a number of issues raised in your response so let’s take them one by one. My first point regards your claim that one of the reasons for not making a distinction between the need to treat a fish in isolation as opposed to treating the whole tank is that such a statement would call for interpretation. Namely, each aquarist would have to evaluate a given situation and decide whether a fish should be isolated for treatment or whether the whole tank should be treated. This, you claim, could be dangerous if the aquarist is inexperienced as it could potentially result in pathogens in the main tank remaining untreated and harming the fish. My response to this would be to look at the potential dangers of avoiding to make the distinction versus the potential dangers of making it. In your statement you claim there is a probability, but not a certainty, of infection. As a result of not making the distinction fish which do not need medication will be unnecessarily exposed to it, with potentially negative effects on their health. On the other hand, less experienced aquarists are deprived of the opportunity to learn the differences between various types of diseases and pathogens. Finally, by following the advise, beginner aquarists may put some of their fish in extreme danger, if they happen to have in their tanks species which should be treated separately due to, for example, their lower tolerances of active ingredients. The danger is multiplied if non-isolation is coupled with overdosing.

Against these, the main concern, if a distinction were to be made, is that less experienced aquarists may isolate a fish for treatment when they should in actual fact treat the whole tank. What would happen then? Pathogens will affect other fish in the main tank. Well ok – in this case there will be further symptoms in the tank. Wouldn’t that alert the aquarist that there is an epidemic and trigger an immediate tank treatment? In conclusion it seems to me that it is preferable to advocate the distinction with the necessary provisos, checks and alerts.

George: Although in theory you are right it seems to me your response is ignoring some “hard” facts. In practice neither the novice hobbyists not their fish may react in the ideal way you respond. Thus, if things were ideal, a novice hobbyist should read all the time, ask questions, have pristine water in his tanks etc. Under these circumstances, it is highly improbable that he will need to medicate before he knows enough to make a correct diagnosis or – at least – know whom to ask for advice. In reality, things follow a completely different pattern which starts with the complete absence of a dedicated hospital / quarantine tank. This in turn makes this discussion academic.  

Marina: Not quite. In fact, this is one more reason to recommend having a hospital tank. Under the circumstances I feel the least one could do is modify the original statement to indicate that treating the whole tank is, on certain occasions, undesirable, and should only be followed if there was an immediate need to treat and no hospital tank available.

Your second point is that only a relatively small proportion of hobbyists are  proficient enough to be confident that on a given occasion they should treat the whole tank or  […] isolate [..] a fish”. Further, that the criteria for doing this correctly are unclear. In response I would point out that there are hobbyists that have a sound, even if empirical, understanding of when one needs to treat the whole tank or isolating a sick fish is sufficient. Say, for instance, a fish develops fin rot. Fin rot is a secondary infection, which the fish developed due to stress. The other fish look fine. It would be an overreaction to treat the whole tank just for this isolated incident. The first thing to do is to isolate the fish and treat it while monitoring the other fish. If, however, the fish had velvet or ich, which are highly contagious diseases and are transmitted via the water, it would be rush not to treat the whole tank. So there are criteria by which to judge when it is more appropriate to isolate, one of which is the type of infection observed. Based on discussions I have had over the years I believe this to be a well established view amongst experienced aquarists.

Now, is it possible to isolate the fish with the fin rot and still get an outbreak of fin rot in the tank? Yes, under certain conditions it is perfectly possible. Needless to say most of these conditions, such as deterioration of the water quality,  will never be an issue with more experienced aquarists. Even in this case, however, the initial action depends on probabilities. Namely, as it is not highly probable that there will be an outbreak, you don’t treat (unless there are sure signs of an outbreak). In other words, you don’t treat just in case!

George: I do agree that the first course of action should not be to overdose. So much so that in the article it is stated that the best option is not to treat at all. It is also stated that most of the times a good hygiene, good quality food and excellent water quality is the best medication. Hence my conclusion, “nothing is better than everything”. Having said that my aim in the article was to address situations which fall outside the boundaries of good general care. Now to exemplify your concern you gave the example of fin rot. As most diseases we come across are not as “clear -cut” as an isolated case of fin rot I suggest we consider something more complicated instead. Suppose a serious fight takes place and results in two fish with torn fins or cloudy eyes.  What do you propose? To put them together in a hospital tank (thus having only one survivor in the end)? To have two hospital tanks? Or treat the whole tank? 

Marina: Let’s sum up so far. We fully agree not to use medication except if it is absolutely necessary. We also agree that a carefully selected good quality varied diet with vitamin and mineral supplements, lots of clean, well oxygenated water with as little additives as possible and a near-nature tank environment with plenty of space for the fish to swim and hide will prevent disease outbreak in the majority of cases. Such an environment will also address some “symptoms” which may be due to deficiencies rather than infections. This then leaves your question about the “serious fight” situation. Well, when I found one of my female L25s with ½ her pectoral fin missing (as a bigger female had bit it off during a fight) I left her untreated in the main tank and she healed fine by herself. I have followed the same course of action for other major injuries of different fish. I have used a hospital tank in cases where the injured fish was unwell and distressed because the others were getting to it. Simple isolation, without the addition of medication, has been sufficient to allow the individual to recover.

I think it is important to emphasize that just because two fish fought and they look a bit under the weather it doesn’t mean they will get ill too. In nature fish fight all the time and they make it back to health fine. It is therefore preferable, in the first instance, to leave them in the tank and monitor them. Even supposing there was a disease outbreak, chances are they wouldn’t both get infected except if other conditions apply, such as the water in the tank is of rather poor quality, the fish are generally run down due to malnutrition or inappropriate living conditions, there is bullying in the tank etc. If the problem was with the water then by rights I would treat the whole tank. If one developed fin rot because he was distressed and his defenses were down, then I would move him in the hospital tank. If both developed fin rot because they are a shy species and they got bullied following the fight as the others sensed they were run down, I would move both together in the same hospital tank. Don’t forget that when they change environment, fish are getting shy until they get to know the new environment. As, in addition, these fish would also be unwell, it is highly unlikely they would start fighting straight away in the hospital tank too. By the time they felt confident enough in the hospital tank the treatment would be over.  So, it all depends on the situation.

Having said that, I do agree that there are cases where a hospital tank will just not help. For instance, under certain conditions different fish in the tank may develop the same primary but different secondary diseases. These cases are rare and are mainly due to a combination of poor water quality and insufficient care when introducing a new individual in the tank. Regrettably, in situations like this very little can be done whether by means of overdosing the whole tank or isolating. More than likely one has to select which individuals they should try and save and forget about the other fish, or even eliminate them to eliminate the danger of possible additional infections.

George: The example with the two fighting fish was used to indicate a situation in which you may have to treat more than one fish at the same time. You don’t treat fish which have lost a scale or show a foggy eye unless you see the situation getting worse by the day and so do I. As I noted in the article, most of the times we treat for the wrong reason. In short, treatment should be the last resort, but I think that treating the whole tank makes more sense in a number of cases. Not always, but not once in a lifetime. The more experienced the hobbyist becomes the more knowledge he has to distinguish each case and take the appropriate measures. However, until one is able to tell a sick fish from a healthy one, I think it is safer to treat the whole tank.

Conclusion: In most cases, isolating the fish and medicating it individually is the right course of action. However, if an aquarist is unsure of the type of disease their fish are suffering from or lack a hospital tank, it may be safer overall to treat the whole tank. It should be noted that in most cases, a large water change, good quality varied food and correct water parameters will prevent the initial appearance of symptoms and/or help the fish recover faster than using medication.


Marina: In the article you advocate overdosing, namely administering more than the label of a given medication indicates the correct treatment to be. You state that overdosing could be anything between 20% and “in severe, hopeless cases, as much as a 1000%”. Is your article addressed to specialists only? Don’t you think that this advise, given to non-experts or even beginner aquarists may be extremely dangerous as it definitely calls for “interpretation”, which was your main concern on the issue of isolation? On what grounds is a non-expert aquarist expected to know how much to overdose by, when to overdose and which case is a “severe” and “hopeless” one? If, for the sake of the argument, we consider other things to be equal, it still seems to me that this advise calls for more than an experienced aquarist to interpret it and apply it correctly as it requires in-depth knowledge of a range of disciplines such as physiology, immunology etc.

George: The article is addressed to all hobbyists and suggests that you have to know what you are going to treat for, you have to know which fish and / or tank you are going to treat and then (if you really have to) you should overdose. I don’t give this advice as a “must do” option. It is what I do in very specific cases and is supposed be an example of how far you can go to save a fish which otherwise would die. For instance, in some cases, like systemic bacterial infections which were noticed too late, overdosing is the only thing that may save the fish. In this case, I would also advocate the isolation of the sick fish. I could argue a lot about the term “medication” and the instructions that come with it, especially in aquarium treatments. You see, with humans things are different and you get a complete insert which covers everything, even remotely possible combinations. With fish, you don’t. As an example, a well known antibiotic gets neutralized (and thus, useless) in the presence of calcium cations. In the human medication it is listed to avoid the concomitant administration of this particular medication and any food containing calcium, such as milk. The same active ingredient exists for fish treatment. I have never seen any warning about this fact. I have never seen anybody suggesting that if your water is very hard then you should increase the dose. I can list many such ingredients which need to be dealt with in a very special manner. However, the aim of this article was not to help the reader get a Ph.D. in pharmacology but to provide some simple principles which s/he must know. Thus, I decided to say two things. First, do not treat at all most of the times. Then, if you have checked the list and you are absolutely sure that you must treat do it the correct way. In my opinion, this means overdosing – I think the example with the antibiotic explains a lot.

Marina I would agree with most of this wholeheartedly. I am still very uncomfortable with the statement that dosing the correct way means overdosing on every occasion. Maybe this is so in particular cases as you indicated, in which case I would be happier if we were to look at the cases where this is applicable. In any case even this statement doesn’t negate my argument, namely if unsure, underdose! Admittedly, by rights, it should be if unsure don’t dose at all. However such a statement is a non-starter for a variety of reasons. One of them is, in your own words, that if an aquarist sees their fish suffering “doing something is better than doing nothing, which is exactly the theory most of us usually follow”. Suppose an aquarist notices a lump on a fish. It could be anything, ranging from a non threatening sarcoma to a symptom of some kind of a nodular disease which could be viral, due to parasites, worms etc. We agreed that more likely than not, most aquarists would treat. Can they possibly know what they are treating for? No, at least not without specialist examinations such as biopsy, histological tests etc. What would then happen if they overdosed? The fish’s body would have to cope with an additional stress, exposure to large amounts of medication, while it is already, potentially, in a weakened state. Could that have a negative effect on the fish? I think so.

You then get other cases, such as secondary infections. Suppose a fish is affected by Pleistophora hyphessobryconis (Neon Tetra disease). This goes unnoticed for a couple of days as the only symptom the fish manifests is listlessness at night and a tendency not to swim with the others. In the process the fish also develops a secondary infection, say fin rot or bloating. In this case, fin rot or bloating is the disease with the most obvious symptoms to the majority of aquarists. Can we know at this stage what the fish is actually suffering from?

My point here is that there is an alarmingly high number of cases in which we don’t actually know what the fish are suffering from. We speculate, and some can have more educated guesses than others. In this context, as you expect an aquarist to know what they are treating for, the advise to overdose is technically not applicable. In practice, however, as we may think we know what is happening, we will overdose again with the wrong medication. Which, in addition to the dangers highlighted above, will also present further complications if later indications compel us to change course of action half-way through the first treatment.

There is yet another aspect to this, which relates to people new in the hobby. A beginner aquarist who sees symptoms of disease in the tank is more likely to get upset than a more experienced aquarist, who has successfully dealt with disease before. Now suppose this person diagnoses dropsy (or thinks / is told etc that the fish is possibly suffering from dropsy). They are under duress because they know the fish is suffering and they have to do something to help. Under the circumstances even the soundest judgment can get clouded. So what happens? They get a commercial preparation (more often than not the criterion of selection is local availability or the advise of a sales assistant who may or may not know about medications and the fish) and they overdose the aquarium with it. That doesn’t even take into account what types of fish are in the tank, which are their tolerances to active ingredients and so on. The variables in cases like this are so many that the most likely outcome is that the treatment will have detrimental effects for a number of individuals in the aquarium, some of whom may be totally healthy!

For these reasons I feel that the statement about overdosing needs to be further qualified. For the same reasons I also think that it doesn’t contradict the view “if unsure underdose”. Granted, in the cases indicated, underdosing may not treat fully the disease (which in itself is an issue to discuss) but at least it won’t result in the fish being poisoned by mistake!

George: It is my time to agree with you wholeheartedly. Overdosing is not something I would recommend to less experienced aquarists or people who haven’t read extensively on the issue of fish disease and medications. A list of cases where overdosing would be preferable would be excellent to have. Such a list could include various categories of diseases/ pathogens, general signs to look for, etc. In fact, I have already worked on a list of the most common things to check before even considering treatment. You may want to add a couple of things to it, too. So, the caring hobbyist will have to read this page first, check everything, then go to the disease identification section, check it (this may take some time) and then come back to read the article which started this conversation and – finally – this discussion here. Regrettably, my experience is that not many hobbyists spend sufficient time reading in preparation for a possible emergency. The ones who do, will undoubtedly save more fish than the ones who don’t – but they will be a small minority. Over the years I have been surprised by the number of people who have navigated the site but didn’t show any particular interest in understanding disease and treatment. Instead there was a clear preference for instructions on suggested dose of medication and proposed length of treatment for each disease. I suppose that this was also in the back of my mind when I suggested overdosing. Assuming there is very little interest in fish disease and treatment, making lists of things to look for, special categories, risk factors and the rest is largely pointless for the majority of cases though it would be particularly beneficial for those few who care to read. 

Marina:  There is a lot of misinformation about fish keeping. For all sorts of reasons fish keeping has been marketed as an “easy” hobby. I have been present in discussions where a tank was described as an additional item of decoration in a house. Yet this is so untrue! Fish are probably the most difficult pets to keep taking into account the amount of care and specialist knowledge one needs to have to offer them good conditions of survival, let alone optimal conditions. Now a tank full of problems far from being a pleasure is a sheer frustration to everybody, including the owner. From a different angle, fish are live and should be treated with respect. It is unfair to buy them, then torture them through negligence and ignorance. I would recommend serious reading to any hobbyist and I think this should be part of our joint conclusion.

Now to my next point. To support your case against isolation you also claim that we don’t know much about the immune system of the fish “let alone fish collected in the Amazon and now living in a different country” (p.1) You further state that we don’t really know which “is the effect of water conditioners or activated carbon on the immune system”, which “of the macro- or micronutrients, vitamins, or trace elements [..that] exist in the natural environment (but not in our tanks) influence the efficacy of the immune system and in what way” and so on. Hence your case that infections could occur where least expected, in which case it is better to treat all the population in a tank given a sign of the presence of undesirable micro-organisms.

Admittedly there is very little information on the issues you mentioned. This is particularly true about species which are new in the hobby. In addition, there is a very limited amount of comparative studies of fish living in the wild and fish living in captivity. Further, I believe that to declare with certainty the effects of a chemical on a living organism one needs to monitor its effects on at least two or three generations to check out the way it influences different individuals, genetic traits etc. Last but not least, we know very little about the properties and behaviour of water both as an “environment” and an “element”.  Yet, this holds true not only for water conditioners, carbon and so on, but also for medications! So why administer if there is no need (by not isolating) or, when it comes to that, why overadminister? At the end of the day, fish live happily in the wild, for all we know usually infested with parasites!

I hold the view that in the aquarium our starting point is damage minimization. Rather than trying to provide conditions which are ideal for the fish, we are starting from trying to avoid conditions which are fatal for the fish. That is because we can tell a fatality, but as you said, we can’t know the ideal. As an example, we add water conditioner in new water because without it the fish die (observation) not because in nature there is no chlorine in the water. For the same reason we also discuss and monitor fish tolerances to different environments (pH, temperature and the like). By working backwards, we are acquiring pieces of information and knowledge which allows us to ascertain conditions of survival. We take survival and procreation to be signs of success because that is what happens in nature.

Yet the survival conditions in the aquarium are different to those in nature. In nature fish fight all the time and get their fins nipped off but they don’t die from fin rot – or at least they don’t die at the rate they do in an aquarium. Hence we provide medication to treat fish – but for all we know this medication is only necessary because the fish is kept in captivity. Therefore, it is not something “natural” for the fish and as such it can have all sorts of negative effects on it.

In nature fish rely on their immune system to fight disease. In captivity we provide medication only to the extend necessary to combat situations which arise because the fish are kept in a specific tank. In other words, we put the treatment in to eradicate the offending micro-organism knowing full well that the same treatment may also do some damage to our fish. The obvious conclusions, following the damage minimization theory, are that:

Ø      we shouldn’t use medication if it is not 100% needed and

Ø      we shouldn’t overdose as we don’t know the short and long term effect of a given medication on the fish (let alone the effects of the overdose).

That leaves us with the issue of the offending micro-organism. Your argument is, whatever you do make sure that you get rid of the micro-organism. This, however, is more complicated than it sounds. As every individual tank is a mini-nature (aka system), we cannot possibly know the correct dose which will get rid of the micro-organism and restore the balance in the given system. Micro-organisms also have ideal and non-ideal living conditions, which is why they tent to flourish under certain conditions. This explains why we have conflicting reports regarding the efficacy of the same medication used to treat the same disease in different tanks. Is it the case that if treating micro-organisms which live in non-ideal conditions underdosing would be sufficient? It seems to be, at least for some I know of. So, when treating Ich, we raise the temperature, and add salt in addition to medication, thus making living conditions for the tomites less desirable.

What is stated in the inserts are broad generalities. In some cases the aquarist may need to overdose, in others to underdose (depending on levels of stocking, water conditions, types of fish, general condition of fish, other chemicals used in the tank and so on).

I understand that pharmaceutical companies, to ensure that medications are effective, always recommend the highest possible dose that the fish will tolerate, which may far exceed the lethal dose for the micro-organism the medication is trying to eradicate. The first issue then is which fish we mean by “the fish” in the previous statement. Surely medications haven’t been tried on all the underwater species. Selective testing does in my view allow for the possibility of underdosing being occasionally required.  Indeed in some cases it is highly recommended to underdose as certain types of fish have considerably lower tolerances to some of the active ingredients in medications than others. In other cases fish manifest excellent natural responses to disease. Hence the theory of “if unsure, underdose”. The rationale behind it is to avoid damaging the fish while still achieving the eradication or weakening of the micro-organisms. This would allow for some time to work out a proper course of action. Remember, underdosing is not recommended to experienced aquarists and it doesn’t claim to solve the problem. It is just a temporary solution to avoid potential damage until proper advice is sought.

Further to the above, it would be interesting to have your views on the following topic: would it be possible, in theory, to put just enough medication to weaken the micro-organism to the point where the fish’s immune system can take over?

George : I have heard the argument about nature many times; this may well be the first time I will try to really respond to it. Your argument is that we do things in our tanks which nature doesn’t do for the fish in the wild. This is true, but it is not a valid argument because, unless you only keep fish collected as adults from nature, the fish we keep in our tanks have great differences from the wild ones. Firstly, the microbial load in a tank is hundreds or even thousands of times higher than in nature. This has the direct consequence that all those microbes (especially the pathogens) are just waiting for the fish to become weak (bullied, starved or diseased) in order to establish themselves in (or on) its body. In nature this doesn’t happen. Due to the extremely low microbial count the fish will probably have enough time to heal before a secondary infection occurs. In our tanks it won’t. Secondly, the fish we keep in our tanks are not comparable with the ones swimming in the water bodies in the wild, unless they were captured as adults. For every fish that reaches maturity in the wild, there are thousands which perished because of diseases, starvation, predation etc. So, the ones which make it to adulthood are simply the fittest for that particular environment at that particular time (natural selection). In contrast, fish raised in captivity are not selected this way. So they don’t have the same resistance, ability, colors etc. The vast majority of fish swimming in the tanks of novice aquarists are raised in captivity. I will not even mention terms like genetic pool, interbreeding etc. which lead to tons of other problems – somewhat related to this discussion. So, what applies in nature, may or may not have anything to do with what we should do in our tanks to treat them. As for the immune system coping with the pathogen, this could work with some pathogens and not with others. Once the pathogen is in the fish it is in optimal conditions and you can’t make them really suboptimal unless you use the proper concentration of the medication or even higher than that. That aside, the immune system of the fish is not the most developed thing on this planet and there are many pathogens which it can’t even fight against.

Finally, the immune system is quite more complicated than what most people think. Thus, do you refer to specific immunity (antibody / complement), cellular immunity (phagocytosis) or humoral immunity (cytokines etc.). Who knows? I know of very few studies on that and most results are mixed feelings.

Your next point is that since we don’t know the effect of medication on the fish it would be advisable in some cases to underdose rather than risk hurting the fish by an overdose, which could have severe side effects. I suppose this is true on some occasions. Yet on others, though we may not know the effect of a given medication on a particular type of fish, we do know the effect of the medication on the pathogen. We also know the effect of the pathogen on the fish. Therefore, you do have the information required to make a decision. The same is true for humans. We know that a number of antibiotics will have a serious effect on hepatic function but we also know that if the pathogen is left untreated the patient will just die. The same is true with most non-steroid analgesic medications which may cause an ulcer or seriously attack a present one. Still, we have to administer them. There is always a benefit / risk ratio. After all, all medications are poisons. Yet most of the antibiotics are far more poisonous to the pathogen than to the fish. The difference between the two levels we call therapeutic range – which means we can use it to the benefit of our fish.

On your last point I would point out that the lower level of pathogens which would remain in the blood stream of the fish, even if successfully fought against by the fish on one occasion, will develop a level of resistance to the medication used to eradicate the others. Should a need arise to treat against them in future, this “resistance” will make further treatment with the same medication very difficult. This, in my view, is a particularly undesirable situation to be in.

 In conclusion, in some cases it is better to overdose the whole tank than treating part of it and risk a re-infection by the same pathogen which may have adapted, mutated or become totally resistant in the mean time.

Marina: I do agree on the issue of nature vs tank; this is precisely the point I was making before. I am also concerned that in mixed community tanks fish also come in contact with pathogens which they may never had encountered in their own habitat and therefore may have no immunity against. Hence my statement that medication is used to combat a “tank” situation.

On the issue of information regarding side effects I would point out that we do have some data, not all the data we need to make an informed decision. Medication is administered to keep the fish alive, not to eradicate a population of pathogens. Though in some cases the two coincide, in others they don’t. There could be cases that if a pathogen is kept at bay the fish can survive happily. In other cases the eradication of the pathogen may also lead to the demise of the fish. Suppose you know that a fish suffers from anchor worm (condition 1 in your statement). You also know that if you don’t get rid of the anchor worm the fish will die (condition 2 in your statement). You acquire medication x which will kill the anchor worm (condition 3 in your statement). However you are also told that this medication will kill the fish (this is the missing data in your statement!). You wouldn’t administer, would you? In other words, that data you claim we have to make a decision is partial and incomplete. We also need data regarding the effect of medication on fish.

Now as you said, in the case of humans, we do have this information. Hence we know that an ingredient x may, in your words, “cause an ulcer or seriously attack a present one”. First, we will consider whether we should treat a condition with ingredient x if the patient is suffering from an ulcer or whether we are better off using an alternative ingredient (something we never do with fish). More importantly, while administering medication x we will be monitoring for ulcer!!! Does this happen with the fish? Can we be sure that a fish doesn’t contract a follow up disease – or become susceptible to one, as a result of treatment with a given medication? This is the point we are debating – what to do in the absence of such data!

This point is quite serious in my view. Please note that I am not referring to people who use medication cautiously. I am mostly referring to aquarists who use medication inadvertently on the first sign that something is not quite right with their fish. Medication, in this sense, is used to make up for the clear lack of proper conditions in the tank. So we are talking about the effects on fish which, for the majority of their lives, are swimming in some form of medication or other.

Moving on to the next issue, I take it your point is that by underdosing in certain cases we could be creating resistant strains of pathogens. Do we have any information which pathogens or types of pathogens underdosing could or couldn’t work with? For instance, I wouldn’t expect underdosing to work with aeromonas / pseudomonas types of bacteria as these are known to be extremely stubborn and very difficult to eradicate. In these cases overdosing may well be the answer. It would be very helpful to have a list of types of pathogens alongside their resistance levels to medications.

I am taking for granted your point about “the immune system of the fish [not being] the most developed thing on this planet” and therefore not being able to fight against a number of pathogens. These and other variations have been observed with humans too –two people suffer from the same disease and one dies while the other recovers. There could also be differences in the recovery time. Yet these arguments do not support overdosing. They support isolation and individual treatment.

As for your last question, I was mainly referring to antibodies.

George: The benefit / risk ratio covers this point exactly. There is always a risk, even for humans, even for the most common medications. Those problems have names like hypersensitivity (to the active ingredient of a medication or even an excipient), enzyme deficiencies (which may not allow a medication to be metabolized as it should), impaired hepatic function (the drug may not be transformed into the active metabolite), impaired renal function (the drug is not depleted from the body at the desired rate). We also have differences in weight and blood volume (drug distribution), fat content (which may bind the drug in a desired or undesired way) and many more. In short, the ideal situation would be to individualize the dose for every single human being. Otherwise we make a compromise, which is exactly what we do. Now, if we do that to ourselves, knowing much more than we do about fish, it is easy to understand that we should follow some kind of similar approach for fish, too.

Having said that, I take your point that with humans we have far more information and therefore we are less likely to administer something which can cause serious damage which, in turn, will remain untreated.

Marina: Your next claim is that the point of the article is “not to treat all, but if you have to do it, overdose (p. 2). We agree on the first part of this sentence. What we are debating is how is one to treat if one has to do it. In essence, when it comes to the fish, every aquarist becomes a specialist doctor. We have license to diagnose, prescribe and treat. And this without any qualifications or training. Now experience makes up for some of it. But would you not agree that lack of experience and uncertainty combined with advice to overdose can be fatal for the fish? Remember, the point we are discussing is, if unsure, underdose (where if unsure is the operative statement)

George: I agree. But how do you define “sure” and “unsure”? Furthermore, how do you inform the  hobbyist of this definition, namely in which circumstances they should be unsure and in which sure? Which are the factors s/he should take into account before treating or before deciding how much medication to add? Is there such a list somewhere? Will a hobbyist read it? Is the list the same for all cases? How many “if-then” clauses would such a list contain? If we can get all this information we can then inform the hobbyist. But then, by the time a hobbyist makes up his mind the fish may already be dead or the wrong medication may be in the water! However, in general I could accept this – about “unsure” and “underdose”. But if you are sure, overdose to kill. 

Marina: So far so good then. I would say we can describe “unsure” as follows. If an aquarist:

Ø                  doesn’t know the fish in their aquarium (species, reaction to chemicals, living conditions, age)

Ø                  hasn’t had experience of the disease they are trying to treat previously and / or hasn’t treated it with success.

Ø                  can’t make their mind up about the symptoms

Ø                  hasn’t used a particular medication before, and/or hasn’t had it recommended by an experienced aquarist.

Ø                  Hasn’t used a particular treating technique before and/or hasn’t got an experienced aquarist to assist them during their first attempt

Ø                  Is using additives in the aquarium which are not specified in the insert of the medication they are planning to use and about which they are not 100% sure they don’t interact with the active ingredient of the medication

then in my view they should be unsure of the validity of their planned course of action. Please note that other factors may be pertinent too, but I am restricting the definition of “unsure” to the absolute minimum for the sake of correct dosing.

Returning to your argument, a serious aquarist will never be in a position that you describe i.e. “by the time he makes up his mind the fish may already be dead or the wrong medication may be in the water” for the simple reason that s/he must have read up about his/her fish (which is possibly the most important determining factor in this equation) way before they ever get ill. In addition s/he should know what to do in case of emergency (they will have a well researched action plan) and they will definitely know what additives they are using in the water. In other words, they would be prepared for the crisis, they would never be caught unaware. And even if they did, e.g. they tried a new species and the fish they got were ill straight away, they would know how and where to get advice from experienced hobbyists quickly.

Again, experienced aquarists develop a feel for things like that and this, combined with their knowledge of the basics of fishkeeping always comes handy in a crisis.

Finally, though some of the information we need is currently unavailable, the whole essence of improvement lies in a starting point which gets added to as we learn more. I am not claiming all the information we need is available; yet some is. Well let’s start with using and applying what we have and we can add to it as we go along!

Conclusion: Fish responses to infections and medications vary a lot. Although treating them individually would be the ideal thing to do, this may not be practical or feasible. Relying on the immune system of the fish to combat the infection is a long shot, so medications have to be used. It is advisable to check various parameters before treating though. In case you are sure what you are fighting against, overdose. If not, underdose and seek advice immediately.

If you notice your fish having symptoms often, address your water quality or the living conditions in the tank.

Immunity against Pathogens

Marina You state that “fish don’t necessarily develop immunity to pathogens [...] so, it is better to rely on drugs”. I am out of my depth here, could you provide some additional information?

George: Not every infection leads to an immune response and not every immune response is efficient. This is rule number one for the immune system of all living creatures. A pathogen has to be immunogenic to the host, which means that the host can “recognize” it as foreign to counteract. The immune system gets activated and responds to different “invaders” depending on the fish’s genetic background. If the genetic background doesn’t “recognize” a pathogen as an “invader”, you simply can’t rely on the immune system. Using a medication also means that you rely less on the immune system of the host organism which in turn means that the fish consumes less energy to fight the pathogen and more energy to heal the damages and repair any structures destroyed.

Marina: Do we know which pathogens are not immunogenic?

George: Nobody knows that. We are talking about different families here, not just genera or species. What may be immunogenic to one species may not be to another. An effective immune response is desired but you can rely on it very seldom. Even tolerance to an infection varies a lot between various species. Thus, Paratilapia polleni (a Madagascan cichlid) shows an excellent response to Ich infections, following the addition of salt and a raise in temperature, while the same infection is devastating to the Paretroplus genus which – in some cases – shares the same biotope with it. In a mixed tank – which is very often the case, there is nothing to rely on. The same is true for humans. Some apes can tolerate the HIV virus and never develop symptoms while humans are far more susceptible to it. This virus is a perfect example of how a pathogen may evade the response of the immune system. To keep it simple, the immune system relies on the distinction between “self” and “foreign”. It is educated not to react to the first (self tolerance) and react to the second (immune response). Well, this particular virus just mimics “self”. Pathogens have been shown to use very advanced techniques when trying to survive in a hostile environment; this is why we are about to lose the battle against them. We simply produce new antibiotics at a far slower rate then they learn to resist the ones we already have. They can modify their biochemical pathways so they will not “use” the medication we use, they can hide, transform (mutate), remain “inert” and much more.

Marina: Suppose we treat fish which could otherwise fight the offending pathogen using their immune system. Don’t we partly suppress the immune system of our fish? Surely that could have rather negative long term effects. The reason I am raising this is that it has been reported that, within limits, fish surviving an outbreak of a disease develop partial immunity to subsequent exposures to this disease (Burgess, Bailey and Excell, 1998, p.295)

George: This is a very remote possibility and not likely to happen. In fact, very few drugs have this effect (suppressing the immune system). Those drugs are meant to be used for this reason and are called immunosuppressive. Usually they are used before and after an organ transplantation to avoid rejection of the graft by the host. There are also a few other medications which have been reported to have this side effect but I have never seen one of them recommended for use in the aquarium.

Conclusion: There is a danger related to underdosing and this regards the rate by which pathogens develop resistance to medication. The immune system of the fish cannot always be relied on to fight pathogens. Observation (personal experience), the acquisition of information regarding the reaction of different species to pathogens and medication is essential to ensure successful treatment.

Viral infections:

Marina: On the issue of viral infections you claim “there is nothing [one] can do for viral infections”. Are you referring to lymphocystis, fish pox and the like? It would be helpful if you could elaborate on this point further.

George: Yes and no. There are some infections by viruses for which there is no medication available. You simply rely on the immune system, which may or may not be activated. That apart, viral infections of fish have not been studied to any extend so we really know very little on this issue. For a great number of reasons, viruses are far more difficult to fight since they rely on the host’s cell to survive. There are only a couple of active substances for viral infections and none of them can be used on fish.

In most references, when you check the “viral infections” chapter, you will see that there is only one suggestion for treatment: “disinfection and quarantine”. You will also note that in most cases, if the infection persists, you have to euthanize the infected fish. However, it is not only viruses we know little about. Even for the dreaded (and quite common) Malawi bloat little is known. Is it bacterial? Is it always the same pathogen? Which is the pathogen? It is my understanding that we use this term not to describe the infection by a specific pathogen but rather a collection of symptoms which may or may not have the same etiology. However, we treat every case in the same way. You see what I mean?

Marina: Absolutely. In a number of cases fish are treated without effect. In some of these cases, particularly when it’s a recently caught wild adult fish which happens to die shortly after medication is administered I have often wondered whether it wouldn’t be better off left without.  All the more so when the fish, despite its apparent health issues, behaves normally and feeds well.

George: In cases like these a fish could die two days after it was added to a tank if the medication was not added, or it could have survived. Nobody knows what could have happened if…  I suppose some of these cases, probably fall into the “wait and see” category.

Marina: The question still remains though, how is an aquarist to judge which case (fish) falls in what category

Conclusion: There are diseases which cannot be successfully treated with medications. In these cases the only thing an aquarist can do is to provide the fish’s immune system with as much ammunition as possible to fight the disease and hope for the best. Isolate the ill fish to reduce stress from possible aggression from the other tankmates as well as competition for food and shelter. Provide pristine water conditions and a well balanced, quality diet. Ensure that the living conditions in the isolation tank are as close as they can possibly be to the natural habitat of the fish. Monitor the fish for signs of recovery or deterioration but try not to disturb it (by turning on lights, making noises etc).  Should the fish deteriorate and look distressed or in pain, euthanasia is the more humane option.

You will find below a suggested list of things to consider before deciding on the best course of action:

Ø      Is the diseased fish a new arrival? If so, quarantine and monitor the fish closely prior to adding medication. If the fish doesn’t deteriorate and behaves normally do not add medication. Seek specialist advice.

Ø      Check your tank’s readings (ammonia, nitrate etc). Compare these with the natural conditions the fish should be living in if the fish is a recent addition to your tank.

Ø      Check the fish’s body (including fins, eyes and skin), breathing, swimming behaviour, eating, “social behaviour” (monitor tendency for isolation or bullying behaviour from other tankmates), faeces

Ø      Check your aquarium glass, plants and “furniture” for visible parasites

Ø      Try to identify the symptoms you diagnose here. Look for disorders which manifest all the symptoms your fish manifests or as many as possible. This will avoid mis-diagnosis as in a number of cases the same symptom could have different underlying causes.

Ø      If you diagnose: black spot, nodular disease, Tuberculosis, Neon Tetra disease, fish pox, glochidial infestation, Icthyosporidium, lymphocystis, tumors, or yellow grab isolate the fish immediately in a tank with water conditions as near to those of its natural habitat as possible. If the water conditions in your main tank are very different to these of the ones in the natural habitat of the fish it is safer to proceed with a gradual change in the isolation tank. Observe the fish for signs of distress and seek specialist advice prior to administering medication. In so doing describe as many of the symptoms of your fish as possible (behaviour, feeding, etc) and add photos. Some of the diseases mentioned here (most notable Tuberculosis and Neon Tetra disease) are thought to be untreatable by some and treatable by others. In our view, treatment of these disorders is possible in some cases, although it is always tricky, expensive, laborious and long.

Correct dosing

Marina: In your discussion of the therapeutic range you are claiming that medications contain the active ingredient between the quantity of x and 5x, where x is enough to kill the pathogen and 5x is enough to kill the fish. Here is my question: Pharmaceutical companies surely don’t try the medication on all the species on the planet. In fact, it is questionable if some of them try them at all, as currently aquarium medications are unregulated by government authorities. It is known that different species have different tolerances to various active ingredients. Assuming that fish A will die if exposed to 5x of an active ingredient, a different fish may succumb to a much lower dose, may be 4x or 3x. This means that potentially, if the active ingredient is at a dose of 3x in a medication, dosing correctly (I use correctly here to mean following the instructions on the insert) may already be sufficient to kill a certain species – or cause other damage to fish, such as infertility. Yet, most available medications do not indicate specific tolerances of the various species to their active ingredients. Some medications do indicate the need to dose differently depending on the temperature of water, the acidity or the types of inhabitants of a tank (e.g. scaleless species), but that’s about it.  Wouldn’t that be sufficient reason to underdose if unsure? I am having in mind less experienced aquarists in particular. I assume that more experienced aquarists will have researched the effects of active ingredients on the species they host prior to using them. In any case, they would be members of various fora and before treating they would seek advice on suitable medications. From personal experience I know that some medications simply don’t work with certain species while others have killed the fish as opposed to the pathogen I was trying to eliminate.

George: I almost never use aquatic medications. I prepare my own from human drugs which are much cheaper and I know what is in there. In this case, if I have made a correct diagnosis and if I know the medication will kill the pathogen I overdose and 99 times out of 100 I save the fish. As for your note about the peculiarities of scaleless fish this already calls for an advanced hobbyist to know the difference. I agree that a hobbyist should spend some time reading to find out which is the highest dose reported to kill the pathogen but not the fish. In the case of a tank which will be treated on its whole, the bottleneck is the fish species most susceptible to the medication to be used. If this is only one fish, you may even consider isolating it and treating it in a different way than the main tank. In this case you can still use less drug but proportionally less (e.g. 0.75 x). If you don’t know anything about the fish and the medication then, of course, there is nothing I can recommend. This is guesswork, not fish keeping. 

Having said that, I have had many reports from hobbyists losing fish because they added too little medication or stopped it prematurely! As you can see, there are two sides to the coin. However, you have to say one thing to your reader. You see the problem? We have to understand that there are no strict general rules here. For every disease you may have more than one suitable medication which may act in different ways (perhaps depending on water parameters like GH, pH and temperature), various  fish may show a different tolerance to it etc. In my view, you will end up with so many recommendations that will render them practically useless, perhaps creating more confusion to the desperate hobbyist who is seeking a solution. This is why I support simple solutions and less rules. I know that this is a bit of a compromise but we have to take into account that the aim of the advice is not to be all inclusive but to help the hobbyist take a quick decision. When there is a pathogen in your tank which is already in your fish, you run against time. There are excellent books and internet sources which will give you more than one solution for each possible disease that each species can contract. In most cases, you will notice that they propose a range of doses. My recommendation is to go for the highest one if you are sure you are treating the right thing. If not, go for the lowest one, even below that.

Marina: Your closing statement above sums it up great. Now, having settled that I would also point out, for the sake of the argument, that there is a difference between you, a professional in the field who has access to active ingredients and most aquarists who admittedly don’t have such access. I does indeed take a more experienced aquarist to treat sensitive species, which is precisely why I would like this point to be made explicit in the article, rather than be assumed. Secondly, I feel you don’t have to say one thing to a reader, people can be presented with a variety of options or possibilities. At the end of the day, reality is not black and white so statements can’t be either.

Thirdly, what can be taken as confusing, namely different medications available to treat different diseases, is to me the key to successful treatment. Why are there different medications available if one could do the job? Clearly, different medications are more/less successful in different situations, their side-effects are different and so on. Well, this is what a hobbyist needs to be familiar with, rather than ignore, and ensure that they chose the best medication for their tank and types of fish. Granted, one can’t do this at a time of crisis, but they can do it before – or even after, in preparation for the next one.

Finally, though I understand your point about losing fish due to under treating, I don’t think it quite applies in this case. It is one thing to underdose when you know what you are fighting against and you know what the appropriate dose is and another to underdose when you are fully or partially in the dark.

Conclusion: If you know what is wrong with the fish and you know your medication overdose to wipe the pathogen out in one swift stroke. While you do so you must be sure of the following:

1.      the type of pathogen you are treating

2.      the effects of the medication on the pathogen

3.      the effects of the medication on your fish

It is important to emphasize that we use “sure” in the broader sense of the word. There is no way we can be 100% sure of what we are treating our fish for. The only way to be sure is a histochemical examination or a specific test. We can, however, have sound educated guesses. This is achieved by inspecting fish closely, taking pictures to post to other fellow hobbyists, comparing pictures of one’s fish with pictures in books or the internet and then making an informed decision.

It always helps to make notes on symptoms observed on fish and success of attempted treatments. These notes can be used for future reference, if required.

If you don’t know your fish or the medication and if you aren’t sure what the disease is and if you still have to treat, underdose.  If you overdose the wrong species or using an inappropriate medication you could end up wiping all the fish off and still leave the pathogen unharmed (supposing its a free swimming form) in the water, ready and waiting for the next load of fish that gets in the tank! 

UV sterilizers

Marina: Your discussion of UV sterilization is of particular interest to me as I leave my lamp on all the time. Yet I never considered that as overdosing. Instead, I was thinking of it more along the lines of a preventative measure. Could you elaborate this point further? 

George: By overdosing I refer to using a more powerful lamp than indicated for your water volume and / or a lower flow rate than this lamp calls for. Always read the suggestion for the most resistant pathogen (usually it is the protozoans) and keep in mind that viruses are not affected by radiation unless you use extremely powerful units. For a more detailed discussion this article may be of interest. As you can see the dose required to kill Paramecium, Aspergilus niger or Tobacco mosaic is 20-40 times higher than the one needed to effectively kill a bacterium. This means that you need a lamp 20-40 times more powerful (e.g. 160-320 Watts instead of 8 Watts) or a much lower flow rate (20 L/ hour instead of 800 L/hour). There are further complications of course. For those wanting to read more on the topic I can recommend an excellent book by P.R. Escobal “Aquatic Systems Engineering: Devices and How they Function”; chapter 5 deals with UV sterilization. In any case I think there is a general agreement that most lamps are overrated and actually just useless to the majority of tanks they are supposed to protect. They are also powered with pumps with an extremely high flow rate which makes things worse.

Marina: I am not sure about the UV being “useless”. I think its just certain types of micro-organisms it can deal with, and in certain quantities. Or are you talking about specific brands or systems (combination of lamp with water capacity and flow rate)?

George: I am saying that UV is extremely useful if you take the time to read the dosage emitted, make some calculations, adjust the correct water flow and have enough power. If those things are not met, it is just a waste of time and space.


Conclusion: UV is a good preventative measure which however needs to be installed with caution to ensure it functions at the desired level. UV sterilization will not cure sick fish but will prevent to a certain extend spreading of certain infections in the tank, if used properly.


Sensitive fish & other issues

Marina: As for the scaleless fish, that is one prime example of what I had in mind when I said if unsure, underdose. Though, as you said, it takes a more experienced aquarist to keep some species properly, regrettably, such fish sales are not regulated. What makes things worse is that most pet shops seem to rely on the aquarist to have the knowledge required to host a fish properly while aquarists, for a considerable part, rely on their LFS for information. Misinformation regarding sensitive species is quite common. Now, supposing somebody has loaches in their tank and overdoses what will happen?

I suppose I could have stated my case differently, namely, always check what kind of medication is safe for your fish, including the proper treatment dose. And if you are not sure ask. And if you can’t ask and there is an emergency what to do? If you think about it overdosing may kill a fish or all the fish, underdosing can only not be effective, in which case the aquarist can always have another go. Of the two evils, the latter is by far preferable, in my opinion.

George: You think that you will have another go, but there is no guarantee for that. Overdosing may kill one fish in your tank but will eradicate the specific pathogen and save all your other fish. As you can see, it sounds equally good, doesn’t it?

Marina: Well I would try to save the one fish too, if possible. Please remember I am talking about a whole species, not about a “weaker” individual of a species. Say certain types of sharks, scaleless fish, plecostomus etc. These fish can be saved if appropriate caution is exercised. An experienced aquarist, if they had to overdose, would remove this species from the tank prior to dosing. If the species also needed treatment, they would treat them separately with a lower dose. Why keep them in the tank and overdose knowing that you will lose them? It is like killing them, isn’t it?

You may argue that to save two or three individuals I am possibly risking the health of all. This may be so, but there are certain provisos. Also remember that the fact there is no guarantee something will happen doesn’t necessarily mean that it won’t happen!

Having said that I see the danger you are highlighting. The best course of action, given the situation, is to advise on ways to overdose without risking the health of fish for which the overdose would be fatal should such ways exist. So here are, potentially, our options. Take pathogen Z in fish A. Z needs 4x of a given medication to die. 4x will definitely also kill A (the fish). What would the best course of action be?

  1. Use an alternative medication if available. Sounds perfect but are there alternative medications to use?
  2. (Over)dose to kill the pathogen and hope for the best when it comes to the fish
  3. Euthanize the fish knowing that it will only live more to suffer
  4. Isolate the fish and let nature take its course whatever this may be
  5. Isolate the fish, treat it with the maximum strength dose it can take and see what happens
  6. If the fish makes it, can we safely put it back in the tank or are we risking a possible come back of pathogens? (on the grounds that no obvious symptoms doesn’t necessarily mean no disease)

George: Ok, this is a nice question. I would treat with 3.9X and hope for the best. If the fish makes it, then I would keep it in quarantine for 10 x the incubation time of the pathogen (even if this means a year in isolation) and then get it back with the others. Let’s be honest with ourselves here. The fish in question should be a really valuable specimen in some way (financial, rarity or sentimental value) otherwise, the answer would not be the same. In this case I would probably go for 4.5 X and hope for the best. Either the fish survives or it passes away but I learned (confirmed in this case) something. 

Conclusion: As aquarists we are responsible for the health and well being of all the members of our fish community. Where different species are kept together difficulties may arise if treatment is required. It is highly advisable to be familiar with the sensitive species hosted in one’s tank to avoid unnecessary losses due to high doses of medication. Where possible, remove sensitive species from the tank and treat them separately with an appropriately lower dose, while treating the rest of the fish in the tank with a higher dose.

Yet an aquarist needs to keep a balance between risking the health of the whole community and protecting one fish or species. This occasionally may result in loses. An awareness of this is essential for fish keepers. It is highly recommended to ensure that risks taken with disease and the administration of medication are well weighed for pros and cons with the best interests of all the fish in mind.

Treating fish individually can be a cumbersome and expensive exercise. This should be taken into account when stocking up one’s tank. It is important to ensure, prior to re-introducing an apparently recovered fish in the main tank, that the fish poses no danger to the other inhabitants of the tank. This can only be achieved if the fish is quarantined for a period of time sufficient for potentially latent symptoms to appear (which can vary depending on the species of fish or the type of pathogen affecting it).

Absorption of medications

George: “Another reason to over dose is that, in the case of a fish with an internal infection which refuses food, the concentration of the medication in the water has nothing to do with the concentration of that medication in the infected organ. If the fish takes food, the food absorbs the medication (during soaking) and is subsequently released in the fish's body. In this case, the medication is far more effective and a 20% over dosing is most probably enough. If the fish doesn't take food, the quantity of the medication actually getting in its body is far less and, in most cases, inefficient. In these cases a bigger over dose is a must.”:

Marina: Fish absorb elements from the water via skin and gills – surely that is true for the active ingredients of medication too? Have we got information about absorption rates? It would be interesting to compare the percentage of absorption from skin and gills to the percentage of absorption from food.  Your point about food is very interesting; I always considered the important factor in treatment to be gills and skin absorption.

George: Not really. Humans also absorb some drugs which come in a specific pharmacotechnical form through their skin but not others (such as Augmentin, for instance). Only gels and ointments get absorbed via the skin. Moreover, only a small range of drugs can be administered that way for humans. Why should things be different with fish?

Marina:  Because fish are aquatic creatures (i.e. live in water) whereas humans live in air? That’s what I thought should make the difference. The permeability of the fish’s skin is different to this of human skin. As a result we need to eat to take minerals in but fish can and do get them from the water via skin absorption. By the way, I am not arguing this point, I am merely stating what my impression was.

George: Very few medications get through the skin and even fewer are antibiotics. Usually you have to feed them with that or allow them to take it via their gills. 

Furthermore, we only use transdermal absorption to fight local infections. For systemic infections we use per oss administration or injectable forms. When a fish doesn’t eat (hence we can’t use its blood stream efficiently from the inside) we have to rely on its gills which can only take a very small quantity as compared to the per oss administration. You can compare this with your human drugs administered by spraying the air around you. Will you ever reach the therapeutic concentration? Not even in a million years, which is a fact you also have to take into account. Then comes the renal function. With humans, we can estimate that and adjust the dosage accordingly. This isn’t always the case with fish. Malawi bloat results in a highly impaired renal function and a fish which refrains from eating, so what do we do since most drugs are not absorbed by the gills? Another issue is the benefit / risk ratio. If the fish is dying then the question about dosage is academic. In short, you have to achieve a steady state concentration of the drug in the blood stream. Since you can’t monitor that, you better be over it than below it. Some tissues may also be a burden to drugs (fatty issues for hydrophilic drugs for example). As a conclusion, there is much more to it than what pharmaceutical companies state on the insert.

Marina: Yes, well, we agree on this. All the more so if we take into account that it is not necessarily pharmaceutical companies which manufacture aquarium medications. Aquatic companies do so too, often lacking the specialized knowledge and infrastructure that pharmaceutical companies have.


Now going back to the food issue. Supposing the fish eats. However the food doesn’t stay long enough in the water to get properly soaked. In the vast majority of the cases the fish will polish the food off within minutes of you putting it in the tank. In the case of top feeders in particular, you may be talking seconds. How does that ensure that medication is soaking the food?

Again, if we are relying on food as the major carrier of the medication, why do we put the medication in the water (which gives rise to issues like oxygenation, switching off UV sterilizers and ozonisers, removing activated carbon and the rest) and we don’t just soak the food for 5-10 minutes in the medication prior to feeding the fish?  

George: I agree. This is the best option, if the fish are still accepting food. Now, you calculate the concentrations. You will be amazed to find that you need a ton of antibiotic to get the same concentration in the water but this is the correct concentration for the blood stream of the fish. For instance, if we assume that the correct dose for a medication is 500 mg / Kg body weight and the fish weighs 100 g then the correct dose for that fish is 50 mg. You take a capsule of 500 mg, you dissolve it in 10 ml of water and then you make sure that 1/5th - 1/10th of it is absorbed by as little food as possible. Then you offer this food to your (isolated) fish. This can’t work in a display tank since fish will get varying quantities of the medication (although even this may be better than nothing). Now, assuming the fish has a total blood volume of 25 ml, we have administered 50 mg/ 25 ml of body fluid which means 2 mg/ml. In order to achieve the same concentration in a 100 L tank, you will need 200 g or 400 of those capsules we started with and a fortune. Even if you do that, still the concentration in the blood stream of the fish will not be the same as the one achieved with the food, because the medication is not absorbed equally well through the gills or the skin – if we assume it is absorbed at all. So, if the fish is eating, soaking the food in the medication is the best treatment.  

Marina: Hold on there. This is an interesting point. So – if you are to just soak the food in medication – and skip the medication in the water, what concentration do you use to soak the food in? Just to note that we are talking about bacterial diseases in this case, and not diseases which can be eradicated by killing the spores in the water, such as Ich. So in essence we are dealing with the host fish and not  with the water and the micro-organisms there. As for your argument about cases in which the fish are so heavily infected that they are most likely at a dying stage I can only remark that in my opinion fish are saveable up to a certain point – past this there is no way back and the best thing to do is euthanasia (easier said than done of course but there are cases where one has to be cruel to be kind).

George: With regards to the concentration of the medication I would consult a book (veterinary pharmacology, Physician’s Desk Reference or other) to get the steady state plasma concentration needed to kill the pathogen. There are a couple of “aquatic” books which are really good and are not overly specialized; some of them are listed in the references below. Once this is done, I would follow the procedure outlined above.


Regarding your point that fish is  “saveable up to a point”, all I can say is that this point is movable and gets further and further away as we improve as hobbyists, assuming that the conditions we keep our fish in improve alongside.  In the distant past, whichever fish developed Malawi bloat was dead, no exceptions. I used all kinds of aquatic medications and none worked. Then I used my own cocktails and managed to keep them alive for weeks, even months. Then I used other combinations and higher concentrations and started saving a fish every now and then.  Now I use single drugs at very high concentrations and I save 9 fish out of 10. I have a fish which had a dramatic demonstration of bloat (almost completely rounded), which received two doses of antibiotic - which would make you faint - and is still alive 4 years after the treatment. It is now happily swimming in the tank of a friend, has bred many times and every time I see it, I almost get tears in my eyes. Just another example which shows that there is not one single answer to this problem but you have to propose one thing. Which is it going to be?  

Marina: You know full well you can’t propose one thing. What you propose depends on who you propose it to and what the situation is. I use a very high concentration salt bath for plecostomus who have fainted (plecostomus in some cases faint just before they die – they look dead but they aren’t) because plecostomus don’t like salt. The salt bath gets their body to jerk (as a reaction) and that forces them to take a breath. They are back alive and I can do something with them. I have saved a few like this. Lets go simpler: the standard high concentration salt or Formalin bath to get rid of parasites or the application of local antiseptic on a wounded fish. I have done both on occasions in the past. Would I recommend it to an experienced aquarist? Yes. Would I recommend it to a less experienced aquarist? No. The reason is that by trying to apply the technique a less experienced person is more likely to do harm rather than good. Trying to catch a wounded fish would probably stress it more and make things worse than leaving it alone – while the baths need an experienced person to determine if the fish is stressed, what dilution they will use, how long they need to leave the fish in for, to which fish they can apply it and so on. Similarly, I got a friend who operates on his fish but I wouldn’t even dream of doing it on my own. That’s how it goes.

On the issue of the “saveability” of a fish I couldn’t agree more with you. Yet I think “saveability” can be used in two ways. Firstly in the sense of “current” saveability of a fish i.e. our ability to save a fish at a given moment in time (given our knowledge, access to medication, etc). Secondly, in the sense of “possible” saveability which much depends on the overall knowledge about the species, the pathogen affecting the fish, research etc.

Saveability depends on outlook, philosophy and commitment. Suppose your fish develop X disease and using a medication you save 80%. Has this been a successful treatment? For some yes, for others no. Some will be happy they saved 80% and will continue to use the same medication in future, should there be a need for it. Others will look for a change, using something different, which will enhance their chances of saving 90% or even 100% of the fish. Repeatibility will also be taken into account here, namely the ability of a medication to successfully address X symptoms on the same species every time it is used. Some medications are “hit and miss” in my experience, which begs the question of whether they achieved anything in the first place or not. Now take your example of the Malawi bloat case above. To all points and purposes you experimented with the fish. You considered it lost (judging from experience) and the rationale was that the medication (or dosage) could do no harm under the circumstances, it could only do good. And it did!!! Yet you wouldn’t have tried this “cocktail” if you felt there was a chance to save the fish using another medication – so you did consider the fish a “goner” at the time you decided to use a different cocktail. The “current” saveability of the fish at the time you decided to do the experiment was zero; yet there were good chances of “possible” saveability.

Conclusion: Soaking food in medication is, in certain cases, the most effective way to combat disease. To determine the best course of action on each occasion the aquarist needs to have a sound understanding of the type of pathogen they are dealing with and the way it spreads in the tank.

It is important to keep informed of new developments in the area of diagnosis and treatment. This will enhance the chances of saving fish which were previously considered lost cases. It is worth making a note of medications used and their effects on the species in the tank and sharing experiences with others. This will allow the aquarist to look for more successful treatments when necessary and improve their chances of successfully fighting disease.

Resistant strains of pathogens

Marina: You claim that single cell micro-organisms can develop a resistant strain to an ingredient from one generation to the next. Does this constitute full or partial resistance? Would prolonged treatment deal with it in the first instance? The reason for asking this is that the inserts of most medications advise to repeat the course if the fish are not fully recovered – which I took it to mean that prolonged treatment could deal with partial resistance.

George: Nobody knows how quickly single cell micro-organisms can develop this resistance and this is the main reason we have nosocomial multi-resistant strains all over the world. If we knew what to do, we would protect humans for a start. A prolonged contact of the pathogen with the medication increases the survivor’s chances of developing resistance. If you just kill all the micro-organisms on the spot this can’t happen.

Marina: Wait a minute. If we don’t know how fast the micro-organisms can develop resistance then why overdose?

George:  Simply because you don’t want to bet on the “not fast enough” option. You know those pathogens have survived billions of years and they will definitely kill all of us in the end. They can react faster and far more efficiently than we can.

Marina: Remember, you cannot eradicate all the micro-organisms in the tank

George: Yes you can eradicate a specific pathogen completely. Mind you, not all micro-organisms are pathogens. In fact, very few are. Most microorganisms are either useful (nitrifying bacteria for instance) or harmless. A pathogen is a micro-organism which can cause a disease to the host organism. Ichthyophtherius for instance is a pathogen even if it is not causing disease at a given moment in time. It has the potential to cause it and will do it once the conditions are more favorable. That apart, we should also keep in mind that there is a “critical mass”. Thus, if a very limited number of pathogens enter the body of a fish, the immune system may be able to combat them effectively, even in a non-specific way. This means that the pathogen may be engulfed and killed by phagocytes so the fish will stay healthy. Non specific responses are one of the first lines of defense of many organisms but are only efficient if the initial infection is caused by a small number of pathogens. When we have a sick fish, we have a body full of pathogens which must be dealt with, on the spot. We can’t just risk them be released in the tank. This is also a good reason to keep an eye on the tank and be ready to remove dying fish, before they are eaten by their tankmates (in case we have not isolated the sick fish in the first place). Microorganisms, even pathogens, come in contact with our bodies every day yet we get sick very rarely. However, in a tank, with its limited space and favorable conditions, things are not that good. 

Marina: While you overdose with a broad range antibiotic or antibacterial agent you will most probably kill a number of useful bacteria or micro-organisms too. In addition, except if you use very specifically targeted medication you are probably creating resistant strains to specific amounts of medication in other pathogens, for whom the dosage is not fatal. This surely is not a road you want to go down as it could end up causing major disturbances to your aqua system, resulting, in some cases, in a further disease outbreak. A safer bet seems to be a combination of administering a dose sufficient to eradicate the excess pathogens and relying on the immune system of the fish to fight the rest.  Hence our efforts to keep the fish in peak condition – so that their immune system can be effective in cases like this.

George:  The issue here is not as much what will happen during your initial course of treatment. True enough, it can be “successful” in that the fish remain alive and the symptoms disappear. Yet there is a number of issues to consider here.  

The first one is what happens in the long term should such approach be adopted. Pathogens which have survived the first treatment, have already developed an increased resistance per se. So in a number or ways the damage is already done. Should you need to follow a repeat treatment the usual dose won’t even touch these pathogens - and guess how high you have to go to kill them next time.

Another reason I prefer overdosage for a shorter time to running a second treatment after the first one is that a prolonged treatment with an antibiotic will have more severe hepatic side effects on the fish than a shorter treatment at a higher concentration.

The second issue is our definition of recovery. Is the lack of symptoms an indication of recovery? You know as well as I do that this is a very relative thing. Lack of symptoms doesn’t always indicate a full recovery yet this is the only thing you are able to observe. Is it enough to make a judgement?

Marina:  Yes, recovery in that sense is relative to the given moment in time (when the treatment is completed) and yes it is quite likely that the fish will be vulnerable to a secondary infection or manifest the same symptoms later. Yet I take recovery to mean that for that given moment in time the behaviour of the fish has returned to normal (i.e. what it used to be prior to them getting infected), they are not suffering from the symptoms I was trying to treat and most importantly they are not in direct danger of death from them. Which means, I am giving their immune system one more go at restoring their natural balance eventually (full recovery always takes place at some point in the future as far as I am concerned).

George: Usually, you treat an organism for longer than the actual symptoms show. As with humans, you never stop the antibiotics because you don’t have fever anymore. High fever means the immune system is fighting to cope with the infection. When it is not there anymore, it doesn’t mean that all the pathogens are killed, does it?

Marina:  No, but it means the body is in control of the remaining population of pathogens and given time it will restore its balance. Now there are certain cases in which you treat an organism for “longer than the actual symptoms show” but this is not a blanket practice. It depends on the pathogen and the treatment.

George: Yes, this is why I said “usually”.

Marina:  Which is why I am concerned about an unqualified statement about overdosing. I am not questioning the benefits it can have in certain, controlled, circumstances. All is well and good if one knows the active ingredients, their side effects and the tolerance of their fish to these ingredients. Yet, if this advice is taken to mean do it with all and on every occasion it can be dangerous. I can visualize somebody overtreating inappropriately and ending up losing the fish.

Conclusion: It is advisable to proceed with treatment only when a reliable diagnosis of a disease has been made. Try to avoid unnecessary or random use of medication as this can have adverse effects on your fish and the balance of your tank. Using inappropriate medication, incorrect dosage, or a number of medications in succession highly increases the risk of losing your fish due to toxicity. Desultory or inapt administration of medication also increases the resistance of the pathogens in the tank and makes further treatment impossible or risky for your fish.


Marina Now, my view about underdosing comes from the following considerations:

I am sure you noticed that in the article I claim “if unsure underdose”. The experienced aquarist will know how much to dose. They will have read, researched, got advice or dealt with the disease before. They will also know which medication is most effective with which fish. The issue is, what is the inexperienced aquarist to do? Here are some possibilities:

a)     Some people, not wanting to wait for the full 5 days of a treatment, will empty a bottle of medication in the tank to get rid of the disease quickly (we have all seen this happen). This may well poison the fish – or asphyxiate them.

George: Yes, this is a good point. However, I also state “read the insert”. Then I recommend an overdosing within reason. On the other hand, if I have to choose one of the two risks, I will prefer the ones associated with overdosing.  At least, I will have a dead fish (the sick one) and a healthy tank.

Marina: I am not sure about this. The dead fish will not necessarily be the ill fish. Say for example you are treating a tank with neon tetras, rasboras and a couple of plecos with a solution containing copper sulphide. The ill fish are a couple of neon tetras. If you overdose, (in fact if you don’t underdose) it will be the plecos that die. Simple as that.

George: Point taken. Yet in the case of an underdose we may end up with fish not showing any symptoms and a time bomb in the tank. Have you experienced an Ich infestation in a tank with Madagascan cichlids? I hope you will never see that. Then you will now what the term “no resistance” to Ich means.

Marina: No I haven’t. I have had an extremely bad case of slime some years ago which took me 6 weeks to get rid off. During this time an observer could say that I broke every rule of administering medication, including mix and match and you have it. But: I knew what I was doing and most importantly why I was doing it, I was aware of the risks associated with my actions, I had a good awareness of what my fish would definitely not take I was taking great care to ensure I was providing them with extra special best conditions otherwise to minimize additional stress. Remember, the advice to underdose is addressed to inexperienced aquarists who don’t have this kind of information. 

George: I am not sure if the advice to underdose will not do more harm than good in the long term though. By overdosing, the hobbyist learns to respect the pathogens and be afraid of them. By underdosing s/he tends to underestimate their potential. This is how I have interpreted people’s reactions.

Marina: I see your point but there is a balance that needs to be reached and I can’t see this to be respecting the possible dangers from pathogens more than you respect the fish. Overdosing can have fatal results for the fish if one doesn’t know what they are doing. You pointed out the extreme to my argument (which I can’t quite see as there is the proviso of “if unsure” there but anyway). Now the extreme to your argument is that a novice will put way too much of the right or wrong medication in the water and wipe the whole tank clear of fish while more than likely the pathogens he is trying to treat will be the only ones left if the medication was the wrong one anyway! 

b)     Underdosing may still get rid of the disease, even following your views in the article. You state that pharmaceutical companies will put a quantity between x and 5 x (more than x and less than 5x) of an active ingredient in a medication – if x is known to kill the micro-organism and 5x is known to kill the fish. Accordingly, even when underdosing, there could well be sufficient quantity there to kill the micro-organism.


George: Well, companies will only put what they consider to be 1x though this in some cases can be debatable. The key point here is that the medication's concentration must be over x and definitely below 5x. However, there is a lot of speculation on this issue, let alone the fact that (as you mentioned) there are fish which are more susceptible to medication, in which case, 5x becomes 3x or even 2x. In this case, the therapeutic range is so narrow that you have to be extremely careful when using the medication, any medication. Moreover, a clearly exhausted fish may not tolerate the 5x although it would if it was in prime, or at least in a better, condition. In this case we also have a narrow therapeutic range.  In such cases, it is the doctor (you and I in this case) who will decide if we will go for a 2x or a 3x. What you propose is to go for a 0.75x. They all state to use the recommended dose and the correct duration of the proposed treatment. This sounds like 1x to me.


What does that mean in practice though? Augmentin comes in capsules of 250 mg, 500 mg and 1000 mg. Now, you tell me, which is the 1x dose ? The doctor will decide if you will take 4 x 250 mg or 3 x 1000 mg on a daily basis, correct? If you do the maths, this is exactly 3x for humans, a species we know more about. When it comes to fish, I am afraid that the information available comes from a very limited number of studies with a small number of fish. Under those circumstances, you have to speculate and trust your experience. I hate to say that but in some cases euthanizing a fish may be the best possible solution from a practical point of view. There is always a possibility that by trying to treat a fish we may be creating a time bomb which will be later added back to our main tank. Overdosing has an advantage here. If the fish makes it, the least we can assume is that it is free of the pathogen we treated it for.  

Marina: On what basis will the doctor decide if you are better off with 4x250 or 1x1000 or maybe 4x1000? Age (less for kids), severity of the condition (ailment), general condition of the body, fitness of the person to be able to take the higher dose, other conditions from which the person may be suffering and so on. If you have a mild case of tonsillitis at the age of 12 getting 1000mg Calcipen may just be an overkill … And here is my question: in a mixed community tank – about which the owner knows very little, if anything at all, (because the owner doesn’t even know what species of fish they have in the tank, what the water parameters are etc) how can he determine the dosage? When is he likely to do more damage, if you advise him to over or to underdose? That is our issue. In other words, you know your fish, what they are, how old are they, who is susceptible to what (illness wise), who is shy, what is their behaviour etc. By all means go ahead and overdose if that is what you think is the correct course of action. But if you give this advice out, qualify it, as it will also be read by people who haven’t got the experience and information you do.

George: This is a complicated issue and we look at it from different angles. I care about the pathogen, you care about the fish. If we could combine the two theories it would be perfect.  Is there a way of achieving this?

Marina:  I think so. Before we move on to this, however, let me address some of your points in the previous statement. I am not convinced that companies put 1x of an active ingredient in a given medication. The “1x” that will eradicate velvet in gouramis will do nothing to goldfish. Conversely treat betas or gouramis for internal bacteria and they are fine with the “1x” while your plecos will be dying as if you still had them in clear water.

I don’t profess to know how companies arrange the formula of an active ingredient and I have to admit I have, in vain, tried to contact a number of them in search of info when I needed it. In fact some won’t even inform you of as much as the active ingredient in the medication. Which is why a number of aquarists try to obtain neat active ingredients and administer these directly instead of off-the-shelf medications. I assume that to allow for different degrees of stubbornness of a disease and different immune systems of various fish they use the maximum strength which won’t kill a fish. The question is, what is the most sensitive fish they use as the lower denominator. Otherwise they should have a product per species (or thereabouts).

George: This would be the best solution. You can read about the resistance of a fish to medication and then act accordingly by cutting down the over dosage you planned to administer.

Marina:  But you see this can be done. Most of the times its trial and error and the aquarist has to learn by it. There is always a better way, which is to try to find the required information in articles, books, discussion threads in fora etc  

c)      Even if the dose won’t kill the micro-organism, it will stun it – which essentially buys time to get information. As the fish will still be alive, if treatment with the same medication proves to be unsuccessful, there is still the option of treating with a stronger medication. 

George: What happens if the microorganism is the remainder population of an already partially resistant one which survived a previous treatment? In this case, it is the microorganism which buys time, not you nor the fish. Furthermore, if you need time to get information, you don’t  use any drugs at all. If you use them, it has to be full strength. Otherwise, you may a) not be treating the pathogen which causes the symptoms (which increases in numbers of course) and b) creating a nice resistant strain from every pathogen present in your tank in latent form which would be susceptible to that medication if the drug wasn’t used in a suboptimal dose. In fact, you make a selective breeding of the pathogens. In turn, the generation of a resistant microorganism will have devastating effects (even to other aquarists if plants / fish were donated during the latent period of the pathogen). Last, a more potent drug may not exist (e.g. lactamases). 

Marina: Point taken – but I take it this really depends on the speed with which the pathogens develop resistance, right?

George: They are fast, trust me. Extremely fast.

Conclusions: While underdosing will ensure that your fish are not in danger of being damaged or poisoned by a medication you are not familiar with it still leaves them unprotected, in a number of cases, against the pathogen you are trying to fight. This, in the long term, can have detrimental effects on your tank, particularly if the pathogen develops a resistant strain to the medication you are using and there are no alternative stronger medications to use.

There is also the possibility that the symptoms disappear temporarily, only to appear later in a more severe form. If you have relied on undertreatment and have not followed up the initial underdose with appropriate dosage it is advisable to monitor your fish for at least the period of incubation of the pathogen for further symptoms in the future. Use this time to prepare for fighting the disease effectively, should it re-appear. Avoid moving tank “furniture”, using nets etc between tanks and donating fish or plants to other aquarists for at least the period of incubation of the pathogen you were treating.


Treating a disease successfully involves much more than following the instructions on a commercially available preparation. There are all sorts of factors influencing the success of the treatment, some of which are unknown. Examples include things like

Ø      the successful eradication of pathogens which, if left untreated or are partially treated, can cause serious problems in the future,

Ø      potential effects or side effects of the medication on the fish

Ø      history of the fish (e.g. have they been treated before for the same infection)

Ø      make up of a particular tank

Ø      environmental parameters etc 

It is the aquarist’s responsibility to work out a course of action which will be successful in the specific environment of their own tank. What is important to remember is treating disease always involves a risk for the fish, whether this is due to a pathogen not being properly treated or to the effects of a given medication on fish. The more we know, the more we minimize this risk. But we cannot alleviate it.

Below you will find some guidelines which we feel will assist in treating disease successfully:   

Ø      It is highly advisable to start reading about the diseases likely to affect fish when planning to acquire fish and certainly way before the fish get ill. Get familiar with the types of medication your fish can take and the types of disease they are likely to contract. This will always come handy in a crisis.

Ø      Don’t acquire sensitive species or species that require special treatment given your tank’s parameters if you are not prepared or able to make special efforts to see to their needs. 

Ø      Prevention is better than cure. It is worth minimizing or even eliminating factors which can contribute to your fish getting ill. This involves, amongst others, careful planning around environmental and dietary requirements.

Ø      Don’t start treatment at the first sign of something not being quite right. In most cases it pays to monitor the fish prior to jumping to a quick conclusion and administering medication. Careful monitoring and a tank log will enhance your chances of diagnosing and treating disease successfully.

Ø      No matter how careful you are, there is always the possibility of disease outbreak in your tank. Be prepared. Have a list with symptoms, diseases and medications ready and the basics at home so you can start immediate treatment should it be required. The earlier you start treatment, the more chances your fish have to survive.

Ø      Less experienced aquarists should always follow the instructions on the medication package except if they are acting under recommendation from more experienced aquarists who have successfully treated their fish for the same disease, or under guidance from a specialist.

Ø      Prefer medications indicating the active ingredient and concentration on the label. This way you can check up on the effectiveness of the medication and the effect it will have on your fish. Over the counter aquarium medications are currently unregulated by government authorities; as there are no criteria for testing it is likely that some have not been properly tested.  In turn this means that they cannot be trusted to be efficient and safe to use. If in doubt, try contacting the manufacturer for details.

Ø      Aquarists having sensitive species should use the recommended amount of a given medication. A safe dose is usually ¾ of the full dose. An alternative would be to start with triple the amount of the recommended dose, stay there for a day or two and then go down to the recommended dose, followed by a water change. During the course the fish must be monitored closely for as much as possible. At the first signs of distress a water change must be performed immediately and the dose returned to the recommended levels.

Ø      Keep in mind that fish, like any other living organism, are not built the same i.e. they are not clones. Even fish belonging to the same species may react differently to the same pathogen or medication; more so when we deal with a tank which houses fish belonging to different species, genera or even families. In this case, the aquarist should seek more information. If a fish is profoundly different from the rest, it is advisable to isolate it and follow two different treatments, one for the whole tank and one for the isolated fish.

Ø      If your fish have symptoms often, try to identify the underlying reasons for it. More than likely you will find these in the water quality, diet, compatibility of tank mates, living conditions in the tank and so on. Use  of medication may work temporarily but will not solve the problem.

Ø      In the event of a loss due to illness ensure that the deceased fish is disposed of in a hygienic way. Wrapping it up and sealing the bag properly prior to putting it in the bin is considered to be acceptable. Do not flush the body down the toilet or put it unwrapped in the bin as this may result in spreading the disease to the local animal / fish population with potentially devastating effects. The danger of “inadvertently dosing [a] predator” has also been noted in cases where a deceased fish has been medicated prior to passing away (Burgess, Bailey and Exell, p. 131)


In short…

If you know what is wrong with the fish and you know your medication overdose to wipe the pathogen out as soon as possible.

If you don’t know your fish or the medication you are planning to use and if you aren’t sure what the disease is but you still have to treat, underdose and seek immediate advice.


Method of Treatment










Treating the whole tank

·         Easier for the aquarist

·         No requirement for additional (quarantine) tank

·         No stress to fish from chasing and moving them

·         Ensures effective eradication of offending pathogens from the tank / peace of mind

·         Minimizes the danger of other members of the community being infected




·         Will medicate healthy fish

·         May affect aquarium balance (e.g. plants if salt is added or the lights are switched off etc)

·         May result in poisoning species or individuals intolerant of the active ingredient of the medication

·         May be ineffective due to tank mates aggression

·         May adversely affect the biological filtration of the tank

·         Has limited flexibility in terms of changing course of action / administering a dose to treat a specific individual etc

·         Can be expensive

·         Very difficult or impossible to administer some treatments (such as local antiseptic)






Isolating Sick Fish


·         Avoids tank mate aggression and competition for food / shelter

·         Once in isolation, minimizes stress of sick fish

·         Enables better quality overall environment / near nature conditions

·         Enables careful monitoring of the sick individual

·         Enables personalized treatment / appropriate dosage

·         More economical

·         Easier to create conditions enabling the medication to be most effective

·         Minimises danger of ill fish dying in main tank and being eaten by tank mates thus spreading the infection.


·         Stress from chasing / moving environment

·         May result in pathogens not being treated in the main tank if other fish are not showing symptoms



·         Ensures complete eradication of pathogens without relying on the immune system of fish

·         May “save” a fish which is otherwise certain to be a casualty.


·         May result in poisoning species intolerant to the active ingredient of the medication

·         May have side effects (e.g. on the reproductive system of fish, functioning of other organs etc)






·         May treat effectively the disease in some species thus minimizing dangers associated with the administration of chemicals.

·         It is easier to change course of action should there be a misdiagnosis, as a lower dose of the chemical is in the water

·         Main disadvantage can be addressed if advice is sought quickly (i.e. more medication may be added at a later stage during treatment once diagnosis is confirmed)


·         May result in creating resistant strains of pathogens

·         Will not treat effectively most diseases in most cases and may result in risking the health of the whole community of fish

·         May result in needing to use stronger medication later, which may adversely affect the fish


Recommended Reading:

Chris Andrews, Adrian Exell and Neville Carrington "The Interpet Manual of Fish Diseases". Salamander Books Limited, UK, 1988.

Paul V. Loiselle "The Cichlid Aquarium", Tetra Press, 1994.

Harford Williams and Arlene Jones, “Parasitic Worms of Fish”, Taylor and Francis, London, 1994

Edward J. Noga "Fish Disease, Diagnosis and Treatment", Mosby-Year Book, Inc, 1996.
Dick Mills "Tropical Aquarium Fishes", Tetra Press, Salamander Books Limited, UK, 1997.
Gerald Bassleer “Colorguide of tropical fish diseases”, Bassleer Biofish, Belgium, 1997

Burgess, Bailey and Exell, “A-Z of Tropical Fish Diseases and Health Problems”, Ringpress, Surrey, 1998

Terry Fairfield, “A Commonsense Guide to Fish Health”, Barron’s Educational Series, NY, 2000

The MERCK Index – “An Encyclopedia of chemicals, drugs and biologicals”, Twelfth edition, published by Merck & Co., Inc., Rahway, N.J., USA, 2001

Physician's Desk Reference (PDR) Medical Economics Company, Inc., 58th Edition, 2004.

Roy P. E. Yanong, “Use of Antibiotics in Ornamental Fish Aquaculture, http://edis.ifas.ufl.edu/BODY_FA084

More specialized books, focusing on treating disease of a particular species are also available, e.g.

Dieter Untergasser, “Discus Health”, TFH 1991

For further suggested reading also look at http://www.waquarium.org/MLP/root/html/SuggestedReadings/FishHealth.html

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