I had this happen once due to low oxegen levels. i just added a airstone to the tank and within an hour everyone was happy. but the other things people told you id defintly take into consideration. I clean my filter often to but i only rinse it lightly to save the benifital bacteria because that what breaks down the waste. things you can do to keep them healthy is do not over feed and do periodicle water changes and cython up the waste off the bottom. this will help keep good water quality. remember to keep 0 amonia and nitrites in your tank. any trace of those can kill fish. keep nitrates as low as you can.