06-01-2009, 10:53 AM
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1077's advice is good, that happens. But I suspect something else here.
Your tank pH you said is 6.9 which is slightly acidic. What is the pH of the tap water that you used for the water change? If it is above 7, I suspect ammonia poisioning.
You have a small water volume with 16g, so any adverse change will be very stressful on the inhabitants. By neglecting your water changes for 3 or more weeks, the tank water slowly became more acidic. In acidic water, ammonia converts to ammonium which is harmless to fish. If you suddenly change a significant amount of water and add fresh water that is on the alkaline side (above pH 7) the ammonium in the tank converts back to ammonia and the fish suffer. The symptoms you cite could be due to ammonia poisioning, so this seems likely. As I said, in a relatively small volume of water such changes can have much more impact.
Nitrite probably won't be a problem, since ammonia and ammonium are consumed by nitrosomonas bacteria and become nitrite, and the nitrobacter bacteria will continue to handle it. However, it is also true that at an acidic pH the nitrogen cycle does doesn't function as well, although the pH usually has to fall closer to 6. But after three weeks with no partial water changes, that may have happened.
Partial water changes should be done every week without fail to maintain a relatively stable environment.