I am not very good with diagnosing sick fish as I hardly have any experience with them. But I can take a shot.
I think there are two possibilities here. First off, is you tank cycled? Are there any detectable ammonia levels? If so, this may be the problem. Ammonia can easily burn the eyes, gills, and fins of a fish. If your ammonia is fine, then I would suspect "Popeye" or similar bacteria infection of the eye. To treat you would have to QT
the fish and administer broad spectrum antibiotics.
I don't belive it to be ich. Looks way to large and I don't see any other indicators for ich on the rest of the body.
That is really all the help I can offer. I am sure someone else who is extremely knowledgeable in this area will jump in (Steve-S hint hint)...