Fresh water fish aquarium?

Okay, here's my dilemma, I just spent $50 on fish and 3 out of 4 are dead within a 36 hour period. We set up the 10gal tank on Sunday and let the filter run and the heater and we conditioned the water. We waited 3 days before we put fish in it. We let them float for 30 mins before we put them in. We cleaned the rocks by letting them soak in water about 20 minutes, we dont overfeed them, the tank is not dirty. They didnt kill eachother. They looked fine. We are now down to one and he looks fine, but so did the others before they kicked the bucket so Im sure its just a matter of time before the last one croaks! Help please, where did we go wrong?