Jazmyne: I'm pretty sure that his disease simply came from the shop, it's a very common occurence |: Otherwise, this kind of infection is usually caused by bad water quality or bad nutrition.
Just to be safe for your next fish, in case this one dies, maybe you should empty the tank and start over? Disinfect it with bleach to kill the bacteria
Then you'd have to start over with the nitrogen cycle, but that shouldn't take more than a few weeks! How big is your tank? And do you know about the nitrogen cycle?