He was either bit, or your water is bad...In either case, you will want to do daily 25% changes, make sure to use dechlor, no need for salt or meds. Change water until you see nothing wrong with his eye. And even after that, welcome to the world of doing massive water changes, in order to keep the fish we all like, in relatively small tanks. Even a 500 gallon for a single convict is "smaller" than his available space in the wild. I am being pretty dramatic, but I think you get the idea...Take a deep breath, relax, and update often....Other people will chime in with anything I missed. Good luck.