The first thing I'd suggest is doing a round of tests. What are your ammonia, nitrite and nitrate levels? Sounds like that may be the source of your problem, but it's impossible to tell without more details. You'll need to give us info on what you have living in the tank, tank dimensions, equipment (do you have a skimmer, if so which one?), what water you use, how much you feed, etc etc.
Try and give as much info as possible and people will try and help.
