Sailfins and all Tangs are herbivores and they're natural "grazers" so he'll constantly be looking for algae, etc. to pick off of the rocks. You should feed him algae sheets with garlic extract at least every other day. I feed my tangs some algae sheets every day, but I also have alot of Tangs and I don't have any natural algae growing in my tank.
And since everyone is going to ask. Is the sailfin in a 27g or 14g tank? If so, you need to take him back to the store. He won't last very long in there.