This sounds like either a serious disease outbreak, or an un-cycled tank. Basically, fish produce ammonia. This is a highly toxic chemical that is also formed by rotting organic waste, such as small bits of live plants or fish poo. Changing the water alone is not enough to remove all of the ammonia. We need bacteria to do that. These bacteria are called nitrifyers and they live in the filter. They 'eat' the ammonia and turn it into substances that aren't toxic to fish. If they aren't there, the ammonia will build up in a tank and will kill fish very quickly. Likely signs of ammonia poisoning include gasping at the surface, stillness or lethargy, refusal to eat and clamped fins. Ammonia stresses fish and may therefore make them more likely to get diseases, and many fish poisoned with ammonia die of disease. How to stop it happening again? Cycle your tank. The best way to do it is 'fishless cycling'. This is where you add ammonia to the tank from a bottle to allow the nitrifyers to establish before you put fish in. The method is long and complicated and explained on the 'New To The Hobby' board here: www.fishforums.net The other way to cycle a tank is to add a filter supplement (bacteria in a bottle) and then stock very slowly, adding one or two fish a week until the tank is fully stocked. You should never overcrowd a tank. If your fish died of a disease and not because of ammonia poisoning, there was probably very little or nothing that you could have done. These things sometimes happen. I lost four of my bettas last week to a disease and I'm having to bleach most of my equipment and isolate some of my fish to try and stop the spread.