That's the only thing I can guess. It dosen't fit any photos of disease that I've seen since the red is not actually on any part of his fins, just between the dorsal and tail fins, and it is all the way around his body.
I don't have any fish that are large enough to that large of a bite, I do have six Serpae Tetra which are known to fin nip. I've never seen any aggression outside of the school, but that dosen't mean it dosen't happen. But like I said, I don't think their mouths are large enough to fit his entire body in, but maybe...
Aside from the apparent injury, he acts normal.
The only other fish are four Bronze Cory and two Oto Catfish, they're harmless.