how can you be positive this fish has ich when 1) you havent seen pics of said fish and 2) hippo tangs are prone to discoloration in general (i.e. they will be pale when the lights first come on in the a.m.)
did you qt this fish? if not, are the discolorations raised white bumps that look like sand granules stuck to the fish, or does it just look like pale area's. what else is in the tank?