My yellow neon tetra fish just died!?

It might have something to do with improper cleaning of the tank, or the fish couldve been sick when you got them. when cleaning anquariums or aquarium equipment, i recommend using vinegar. it wooks rather well and it is natural, so if there is trace amounts left it wont harm your fish. i would go to walmart and get a cheap 1 gal tank and airstone if u dont have one. clean out your 20gal again with a vinegar solution. keep your fish in the 1 gal while doing this, then return the fish that are acting normal to the 20 gal. keep an eye on the fish in the 1gal for a couple of days. also check your water conditions. you can add a bit of aquarium salt to the 1 gal and that will probably help. you should use 1 tblsp for every 10 gal so only a couple of pinches. return them to the other tank when they start acting normal. good luck