[George]

Any pictures?
If I have to guess, it's a bacteria infection of internal organs of some sort. If it's my fish, I would get it out of DT put it in a QT and treat it with antibiotics. (don't dose antibiotics in your DT!). Then again, some people here think that netting a fish out stress the hell out of a fish. (I personally don't believe that). You can leave the fish there and hope for the best. Besides, antibiotics for marine fish are really a shot in the dark based on my experience because there are so many strains of bacteria and who knows if we have some kind of "super bug" that is resistant to antibiotics. But again I prefer to at least give the fish a chance. Bacteria infection is very serious for a fish.
Good luck!