He has a kinda slimy coating over him?
And he is looking rather white?
Then it's Ich
Believe it or not ich is always present in a fish, but just when the fish is stressed,like being moved from tank to tank, that's when it shows up.
To lower the rate of stress on a fish and to find the best, most healthy fish I would do one of four things
1.) find someone local to you who has wholesale fish, or who is a diver in coastal waters
2.) Get your divers license and a fish retrieving permit, would take a long while, but I have mine, and my life's never been the same
2.) don't buy from chain stores, because the fish they receive have been transferred from place to place, tanks to tanks to much. (Petco)
4.) Keep water parameters in check