Your description is very non-specific. Do the fish die with their jaws wide open? I've read that is a consistent sign that they died of suffocation, but I'm not sure that they always show that symptom.
Do the fish die at night, or during the day?
Have you had any leaks of the yeast mixture into the tank?
Are the plants growing well?
Have you checked the circulation? Is the filter running well?
20 fish in a 40 gallon tank is not a real small bioload. Depending on the size and nature of the fish and the amount you feed it could be a high bioload. That's about the highest load I keep. I like low bioloads.. My 55 gallon tank has 9 fish. My 150 has 14 fish.
Roger Miller |