Ich usually shows up in the first week of a new fish being introduced into a tank.
Do you have any invertebrates, corals, or anemones? Because if you don't I would recommend adding copper or doing hypo-salinity. If you do I would probably just make sure to get him fed good, keep the stress low, and make sure water quality is top notch. In the ocean fish have about a 50/50 fight off rate of ich.