Whats wrong with my oscar fish?

my oscar fish is sick when we move him he keeps swimming around crazy and bumping into the walls, and when he gets to the bottem of the tank he just stays there and lays on his side and wont move, hes also not eating, we went to the pet store and asked a few people about an they said to get the water tested so we did and they said that the ammonia was high but they gave us an ammonia remover and we put that in, whats wrong with him?