You may or may not know but ich has a 30 day life cycle. Even if the fish is free of external parasites, there is a good chance the ich is still alive in your tank. The ich cysts will fall off the fish, fall onto the substrate and will gestate there for almost a month, then re-appear on the fish. Most encountering ich for the first time think this is a secong case of ich, but really just the same case following its life cycle. Treatment must last this period to kill off all the visible external parasites off the fish and also kill all the cysts in the substrate.
Good Luck.