this one?
Blue caribbean tang (acanthurus coeruleus)
http://www.liveaquaria.com/product/prod_display.cfm?c=15+43+328&pcatid=328
At first glance it looks like a changeling Atlantic Tang, but the juvis have blue trim at the ends of the yellow dorsal and anal fins, which this doesn't seem to. After that, possibly a type of Mimic.