If you go to your fish store and pick up a liquid test kit which tests for ammonia, nitrate and nitrite (API is a good one to get). You can then bring it home and use the kit to test the content of these in your tank water. You should come back with a reading of Ammonia= 0 Nitrite= 0 and Nitrate should have some kind of reading, usually under 20. If any of these are higher, you need to perform large water changes every day and keep testing the water until you get these readings. This will help even if it is not the route cause of the problem, which I suspect it is.
