How long have you had the tank up and running? Did you cycle it before adding fish? How are you preparing your water? Do you have an RO/DI or using tap water? If Tap, how are you removing the chlorine? If you are a fish only tank, I would start by just testing Ammonia, Nitrite and Nitrate. If you have either of the first two you may need to cycle the tank longer. You should be able to pickup any of these three tests at any fish store. Get a high pH kit as well.
Ammonia, Nitrite and Chlorine are Deadly to fish, so its possible high levels of any of these killed your fish. Nitrate is bad at high levels.
Sorry to here about your fish.