Where did you get the fish? A lot of times people who buy fish from chain stores, or even private stores that are more about money than the stock bring him diseased, low quality fish.
Every fish illness has one of two causes:
1) Something the person that had it before you did.
2) Something you are doing.
So if your
ammonia, nitrites, and nitrates are good, and your water's hardness and
pH is fine for those fish, and you've not added any chemicals to the tank that aren't supposed to be there or touched the water with chemicals on your hands, and are not overfeeding the tank, it could've been reason #1.
I would isolate the cory and do increased waterchanges on the isolated cory. I wouldn't use any chemicals until I knew what the problem was. See if the waterchanges help first, would be my suggestion.