Sounds like he may have a bacterial infection, or he is being harrassed by another fish. If the fins are frayed and raggedy, as if they are worn out looking, then it is most likely a bacterial infection. If the fins look like clean tears as if they've been bitten into, then it is harrassment by a tankmate. Spend some time observing your tank closely [but not so closely that your fish realize you are watching them] to see what is going on. Perhaps you will pick up on some sneaky harrassing behavior going on. Don't rule out harassment just because you've had all your fish in the same tank together and never had problems. I had a 3 clowns together with a tang for 2 yrs and they got along very well until one day I noticed 1 clown dead and the other 2 sliced up because the tang sworded them.
If you rule out harrassment, then I would suggest you treat the matter as a bacterial infection. Unless you have a strictly FO setup, meaning no LR, LS or reef, inverts, then you will have to treat in a separate hospital tank. I usually use Maracyn and Maracyn2 for bacterial infections. Take a good look at the fish and see if you notice any reddened gills, irregular breathing, etc.
Good luck.