Why did my fish died?

Sorry to hear about your fish. Since you weren't there when they died, the question is, did they bloat before or after death?

If they bloated before they died, it's likely they had "dropsy" I think dropsy is a bacterial infection wjich affects their kidneys and causes them to bloat. Unfortunately, once they have bloated, their kidneys are irreversibly damaged and they will die. I guess it's kinda hard to recognizethe problem before bloating appears, but the symptoms are there, you just have to observe. The kidneys stop working and they retain the water in their body. I keep maracyn-2 at the house mainly for this problem. One of my son's plecos died from this a couple of years ago, and it took alot of research to figure out this was the problem, but not before he began to bloat, unfortunately :(. Maracyn-2 will clear up the infection before it progresses too far. It's the only medicine I have found that expressly states that disease (and other diseases) on the package. It's rather expensive but worth it if it saves your fish!

It might be a good idea to at least partially treat the rest of the fish with this in case it has spread or to prevent spreading.

Good luck!

fish are very sensitive...(1) you have to make sure all the chemicals in the tank are at the right levels (lots of pet stores will test for a small fee or even free if you just take them a sample of the water) and (2) if you got the water for your tank from the faucet, some cities have chemicals in their water (like chlorine and fluoride) that can kill a fish so you have to get counteractive treatments from the pet store to make the water safe...(3) when you cleaned your tank did you use soap? you can't use any type of soap on anything in a fish tank because no matter how good you think you rinsed it it can still have residue on it. i find the best way to clean aquarium parts is with a steam cleaner... and finally (4) fish give off chemicals of their own that can pollute their water (like humans exhale carbondioxide) so every so often you need to do a partial tank water change (how much you change depends on how big of an aquarium and how many fish are in it, your pet store should be able to tell you how much) i once thought that since eventually the water evaporates just adding fresh clean water to replace that would be enough but it's not. the chemicals that the fish make are not evaporated with the water so they build up and eventually will kill your fish....hope this helps!

It could have been ammonia posioning or lack of oxygen in the water

Bcuz you didn't take good care of it !

Do me a favor do't get another pet other wise you'll kill it. learn how to take care of pets first!