How big? You need to do as big a water change as possible. Chances are that the ammonia is well over 1ppm by now, doing only a 50% water change isn't enough to bring it down to a safe level.
I took the water sample to my local fish store and they tested it for me. They didn't give me the exact details but they said the water was fine.
they also gave me 2 replacement fish for the 2 that died last week so now there are 5 fish in the tank.