I can't tell if his eye is missing or cloudy. From the pics, I wonder if it could be fin rot. I'm no expert, though. I hope you get more confirmation soon. I will say if it looks like more of his fin is wearing away with each day, I would start doing large daily wcs or look into qt tank if that's an option. I also see it recommended to add aquarium salt as part of treatment, but again, wait for an expert because I don't know if that's safe with these guys because I've never had them
It could be from nipping, I don't see any cloudiness/fuzziness. How long have you had them? I'd say if you've not seen a lot of progression it might not be disease. Either way, I hear large daily water changes good for injury/disease Good luck