How long has your tank been established? Are your parameters all in line? Ich is usually brought on by two things, stress and water quality. FEED FEED FEED that fish, and keep those parameters in line. Double check your salinity against your current hydrometer, or refractometer. If it's high, your fish will be more prone to parasitic infection due to the softer water. Can you give us a full rundown of your system? I do mean a full rundown. Parameters, equipment, LBS of live rock and sand, size of tank, water change regimen, etc. Ich is a huge pain once it gets a foothold, but once you figure out the source of the problem, it will usually go away in a few weeks.