After writing hundreds of articles answering thousands of questions and spending too much time in various boards and lists on the web, one might think that I am the guru of fish keeping and nothing goes wrong in my tanks - ever. Well, nothing can be further from the truth. The only difference is that I use to observe my fish very closely so I take notice of any behavioral or appearance changes and thus save some precious time that may make the difference between saving and losing them. Recently, I had another of those moments when you have to decide within minutes what you are going to do. I decided to share this experience with you only because I think it is a very good example of how your quick and correct action can avoid a tragedy. On Saturday I made my usual water change (approximately 30% or 330 liters). As usually, the fish were not fed that day. On Sunday night I noticed one of my fish (the male Chilotilapia euchilus) standing at the corner of the tank and not responding as usually to my presence, while its color was a bit dull. I checked the rest of the fish and there were no other signs of strange behavior so I thought this might be a temporary bad mood of the fish or perhaps it had been chased by another tankmate.

However, on Monday morning the tank smelled disaster. All the small and medium sized fish were at the surface whereas the big ones were lying with their bellies on the sand. Even fish that don't stand the presence of each other were lying next to each other like the two male Nimbochormis. 

All big fish lying with their bellies on the sand. Gasping for air (at the bottom), their colors faded. A clear sign that something is wrong with the water. Immediate action is required. Believe me there is no time - don't leave it for later.

Two male Nimbochromis (livingstoni and polystigma from left to right) and the female polystigma lying next to each other. Something is wrong in the picture. Fish may not speak but you should know how to interpret their behavior.

My first thought was that there had been an ammonia spike because of a dead and decaying fish. I quickly run some tests which showed that NH3=0, NO2=0, NO3=0, GH=14, KH=17, pH=8,3. So it wasn't ammonia or nitrates. The water was full of small bubbles and both pumps were working OK, the temperature was 25.7 C so the data didn't point anywhere. In those cases, a large water change is the first thing that comes in mind. I immediately performed a 30% water change while thinking what else should I do since - clearly - there was something wrong with my water, however it was the same water I was adding again. After dechlorinating the water I decided to add a generous quantity of activated carbon in the filter which should remove any organic substance which might be responsible for this situation. I had to leave for work so I thought that I did what I could and I hoped everything would be OK when I would see my fish again in the afternoon.

 Continued in next page.