How long have you had the fish? Post up some tank specs, particularly all your water readings. Is the fish all right otherwise?
Try doing a large water change in the tank first. If this doesn't work, then the fish will need to be treated in a hospital tank with MaracynTwo for SW fish, double dose for 5-7 days. Just prior to each dose, do a water change and, if you can, run carbon for an hr or 2 before the redose. No carbon filtration during treatment, though, except for just an hr prior to a redose.