Well changing the water probably stressed him out even more, but I don't know a LOT about fish hehe. I do know that aquarium salt is supposed to help sick fish a lot. Otherwise he isn't swimming upside down and like, rubbing against the rocks and stuff at the bottom of the tank is he?? If he is, then that's a disease that... I forget the name of, but when I worked at daycare one of the rooms had an aquarium where this happened to one of the fish, an Oscar I believe. He survived but I forgot what he had.