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pH (Hydrogen Potential)
is chemically defined as
the negative logarithm of the concentration of Hydrogen ions in a solution. This, although accurate means very little to the average fish keeper. We need to simplify things a bit and then examine why pH (and not only) fluctuations may easily kill living organisms.
Water consists of two molecules of Hydrogen (H) combined to one molecule of Oxygen (O) as we all know. What most fish keepers don't know is that a very small portion of water molecules dissociates and gives equal numbers of hydroxyl groups (OH-) and hydrogen ions (H+). It is evident that only equal numbers of OH- and H+ can exist in pure water. Actually in distilled water, only 10-7 molecules dissociate (one molecule of water every 10.000.000 molecules) providing 10-7 OH- and 10-7 H+. The meaning of this is that, strangely as it may sound, water is only slightly soluble in water!!
The second critical factor to remember is that the multiple of OH-
x H+ is constant for water and aqueous solutions. This means, the more H+ ions in such a solution the less OH- in it and vice versa.
The negative logarithm of 10-7 is 7, therefore distilled water has a pH of 7 which is called neutral, meaning that equal numbers of H+ and OH- are present or, in other words there is no surplus of either ions or negative groups.
If you add an acid (by definition any compound that releases H+ when dissolved in water) then the H+ molecules will accumulate in the water. Thus the concentration of H+ will become (as we keep on pouring acid) 10-6 (one H+ ion every one million molecules), then 10-5 (one ion every 100.000 molecules) then 10-4 (one ion every 10.000 molecules) or even more. Of course the pH will gradually change from 7 to 6, then to 5 and 4 or even lower. Any solution with a pH less than 7.0 is called acidic. The lower the value the stronger the acidic character. Concentrated acids have a pH of nearly 0. At that pH only H+ exist.
Exactly the opposite will happen if you add a base (any compound that releases / produces OH-). As we said earlier since the total multiple must remain the same, the more OH- in the water, the less the H+ will be. Thus the concentration of H+ will go from neutral (one every 10.000.000 molecules) to less and less H+ (one every 1.000.000.000 molecules or even less). One every 1 billion molecules means 1X10-9 which means a pH of 9. Any solution with a pH over 7.0 is called basic or alkaline. Strong bases (NaOH, KOH etc) produce solutions with a pH value near 14. At that pH there is no H+.
Why should we care ? For many reasons. Firstly, as we now know, a fluctuation of 1 point in the pH scale means a tenfold increase or decrease in the H+ ions present in the solution. For example, when making a water change with tap water with a pH=6.5 and then you use your chemicals in the tank to bring it back to 8.5 for your African cichlids, you may see some fish die a day after. Why ??
Your cichlids were accustomed to a certain H+ concentration. Then, suddenly, they see new water coming in their tank, which only contains one hundredth of the normal H+. This is a shock by itself, a very serious shock. While in stress the fish try to adapt to the new situation when suddenly something is added in the water which creates a new solution with 100 times more
H+. No organism can adapt to this sort of fluctuations !!
Let's look at what happens in the cells of the fish or plant. I am going to keep it as simple as possible so you will still be with me at the end of it. The cell is like a membrane which is permeable. In short, there is a narrow limit of differences in concentration that the membrane can handle. If the outer concentration suddenly raises 100 times then the cell has to react. How ? By releasing water, so the concentration inside increases, too. Or, by absorbing as much material it can handle in order to level the concentrations. However, the cell is a living unit. It is not a lifeless membrane which can stay intact. There is a limit on how much water it can expel or how many H+ ions it can cope with in the inside (cytoplasm). With fluctuations of this magnitude, most cells simply can't cope. This is even more pronounced if the fluctuation is instant. Cells have a remarkable adaptability to their surrounding and can cope with this kind of fluctuations if they are gradual. A great way to reduce this sort of fluctuations is the use of buffers which have the ability to "absorb" the influence of an acid or a base and keep the pH relatively stable. This is not valid for pH only. The same is true for the GH, KH, conductivity, alkalinity etc. These two entities (GH & KH) show how much calcium, magnesium or carbonates are dissolved in the water. Again, an instant raising of the GH from 10 to 20 will cause too much stress to your fish. The living cell has a certain osmotic pressure in the interior (proportional to the concentration of particles in the cytoplasm) and has reached a dynamic equilibrium with the surrounding
osmotic pressure. The target of every cell is to minimize the differences between its interior and the exterior pressures or keep a specific difference. Obviously, the sudden addition of salt in the environment causes the cell to counteract immediately in order to survive, which means it has to absorb salts at dangerous or even fatal levels. You see, the cell has to survive first and then deal with the extra salts it has accumulated. Again, a gradual increase will allow the fishes to adapt to values that would kill them if applied instantly. Bear in mind that not all species have the same kind of cells, i.e. cells that have the same tolerance or can survive under the same conditions. It is obvious that when cells are exposed to conditions outside their tolerance range they die. For example an African cichlid from Lake Malawi may survive for months or years at a pH of 9.2 with a GH=30 (though not the optimal conditions) whereas a discus will die very shortly!
Rule #1 : Know your fish and the range of conditions they should live in. This is something that needs to be understood. Every single species carries its own genetic code which is information on how its cells should be built. That is nature's way and it won't change because the fish is bred in captivity. Remember that this code was naturally selected as the "best" millions of years ago and still remains the same. These cells (and subsequently tissues, organs and organisms) may adapt to a wide range of external conditions but not without a cost. Adapting means starting, stopping or modifying something, perhaps by "irregularly" activating or inhibiting a biochemical pathway. Experienced fish keepers usually keep fish from the same habitat in their own tank and they try to mimic nature as closely as possible.
Rule #2 : It is highly recommended to
dissolve the total quantity of salts or other additives (e.g. pH buffers) in a
couple of liters and then add the solution little by little as the new water
comes in your tank. This will greatly minimize fluctuations. Avoid any instant
corrections or alterations of the conditions (temperature, lighting etc) and
especially of the water parameters.
update >
Recent discussions with fellow
hobbyists indicate there is some confusion over the relation of KH to pH. Here
is an attempt to clarify matters.
In a "normal"
tank, pH is directly linked to
carbonate hardness
(KH) and the quantity of
carbon dioxide
dissolved in it. A formula allowing you to regulate the KH and CO2
content of your water in order to get the desired pH can be found
here.
This sounds very simple and easy - which it is, under normal circumstances.
However, there are some misconceptions related to these parameters; being aware
of these will help you to avoid mistakes which may harm the health of your fish
and plants.
Most people
believe in the concept of "soft water = low pH, low
General Hardness
(GH) and low KH". Some biotopes call for such a combination (most notably the
Amazon one) so there is a great number of hobbyists trying to recreate it. Let's
look at some issues related to this concept.
To start
with, the terms "soft" and "hard" refer to General Hardness
(also called permanent hardness) and not to KH. This kind of hardness is,
in the main, the result of the presence of Calcium, Magnesium and Iron cations
in the water; the values of these are expressed as carbonate equivalents.
"Expressed as" means that we use the weight of their carbonate salts in order to
make our measurements and identify the GH levels of our water. It does not mean
that what we really have in our water is Calcium or Magnesium carbonate. We may
or may not have it. If we have an equivalent amount of Calcium chloride for
example (CaCl2) then we will have the same GH even though we don't
have a single carbonate radical (CO32-) in our water. This
is extremely important to note. GH has to do with calcium and magnesium cations
and not carbonate radicals, so GH is completely independent of the pH of the
water and totally different to KH. Please note this carefully: GH has nothing to
do with the pH. You may have an extremely high GH and still have an acidic pH -
in fact, you can have any combination of pH - GH values. If you have a mixture
of calcium chloride with hydrochloric acid, you have a pH close to 1 and a GH as
high as you want it to be (even more than 200). In this case you have a very
hard and very acidic water. You can also have a solution of potassium hydroxide
(KOH) which will result in the softest and most alkaline water you will ever see
(GH=0, pH close to 14).
Carbonate hardness
depends on how many carbonate radicals (CO32-) exist in
the water. This is directly related to the pH of the water, since carbonate
radicals make a buffer with the CO2 from the atmosphere (or the one
injected in the tank) so we have a CO32- > HCO3-
> CO2 system. The three entities in this system will interact; if
you raise the levels of one there will be corresponding changes to the values of
the others as a consequence of this rise. For example, if you add some CO2, some
of it will be changed to HCO3- and a small part of that will be changed to CO32-
to keep the ratios of these three entities balanced. Thus, the overall
conditions in the tank will remain stable (up to a point). In simple words, this
carbonate buffering system will keep your pH value stable - within some limits
of course. Even if it is not able to keep it stable, still it will not allow it
to increase or decrease rapidly. It will act as a pillow, making those changes
very gradual. This system, depending on the amount of carbonates in the water,
will "absorb" the changes; this is called "buffering capacity". In short, the
higher the KH, the more CO2 you can add to your water without any
serious changes in the pH of your water. The lower the KH you have, the more
prone your system is to sudden and very significant pH changes (pH swings or
even pH crash). This is very important in systems which employ a supply of
carbon dioxide injection. Thus, people aiming at a very low KH with a carbon
dioxide injection should be very careful because in reality they are always at a
razor's edge. If something goes wrong, there is nothing to stop their pH from
plunging or sky rocketing. There are many things that can have such an effect.
The wrong type of stones and rocks, a sandy substrate full of calcium compounds,
a dead decaying fish, dying plants and many more. It should be noted that a
swing of the pH around the normal value of 7 (I mean a pH value swinging from
6.5 to 7.5 in a planted tank with carbon dioxide injection) has one more
(potentially lethal) consequence. Ammonia changes to ammonium and back. However,
the toxicity potential of the two is not the same (ammonia being much more
toxic) while the environmental conditions in the tank range from safe to
non-safe during the day. I would say that a KH of 4 is the minimum you can be
safe with. Some claim KH=3.3 to be the minimum safe level but I regard 4 as the
minimum – especially for a planted tank with heavy carbon dioxide injection. I
have kept some discus in water with a GH=0.5, KH=0, pH=6.0 for months.
However, I had to change their water every day, there was no carbon dioxide
injection and I was taking daily pH readings.
It should be
noted that KH is not linked to GH in any way. Thus, in the previous
example the addition of calcium chloride / hydrochloric acid will result in the
following water parameters: GH> 200, KH=0, pH=1 and no buffering capacity at
all. In contrast, a strong solution of Na2CO3 (sodium
carbonate) or NaHCO3 (sodium bicarbonate) will result in the
following parameters: pH = 8.4+, GH=0 and KH> 100 with a huge buffering
capacity; just note that the pH will remain over 8.4 even if you have dissolved
12 ppm CO2 in it. Practically, you will have to dissolve more than
300 ppm CO2 in this kind of water just to make the pH neutral (7.0).
Of course, nothing can live in this kind of water, it is just an example of
extremely soft, very alkaline water with a lot of CO2 in it.
The
formula
which links Carbon dioxide, carbonate hardness and pH is a very useful one and
allows you to predict what the final conditions will be. It will directly show
you how much carbon dioxide is dissolved in the water so you can increase or
decrease the amount injected to get optimum CO2 levels.
However, this
formula only works if your buffering system relies only (or, at
least, primarily) on carbonates. If it doesn't, the formula won't work at
all, so you can't use it even to get an approximate estimation. This is
definitely the case when pH-up or pH-down additives are added in your
tank. This is the main reason I never recommend the use of them. Most of these
additives use phosphates to buffer the water at the desired level. As a result
there are two competing buffering systems in the tank; the aquarist can never be
sure which one will take over at which stage. This in itself undermines the
buffering capacity of your water since it essentially relies on two systems. The
same holds true when tannins or humic acids are added, or during extensive peat
filtration. Usually, the effect of those agents is minimal and the carbonate
system is still the major player in your system but when "too much" of the
softener is added, then the situation may change.
In short,
those trying to recreate an Amazonian environment should bear in mind that this
is a natural system which can't be replicated in a tank. It relies on the low KH,
but also relies on tens of other chemical agents which are dissolved in it thus
keeping the pH slightly acidic without any CO2 injection. This
is another important fact: when a fish should be kept in slightly acidic water
this should be ideally done without CO2 injection.
I hope the
above will clarify matters and assist hobbyists in achieving the required water
parameters.
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