If you have no inverts our corals in the tank and just the fish, I would do the low salinity treatment madsalt mentioned. Just use a refractometer as you need precision for salinity for this to work. I wouldn't trust a hydrometer with this one.
Otherwise, if it's eating, just keep feeding it like crazy and soak food in garlic. They get over it usually pretty quickly and some argue the catching and treating it all that time is worst for it because it stresses it out. I had my giant sailfin covered in ich and 7 days of heavy feeding and it was gone. Same for my 4 other tangs, a foxface, a copperband, and about 10 other fish.