Probably injury from fighting with the other tang, but how much damage was done (being it happened in a sensitive area like the eye) and whether or not it gets infected now would be my concerns. Normally I would tell you try feeding vitamin supplements (Selcon, Zoecon, Vita-chem) first and that's still not a bad idea if you cannot QT. However, being how often I see "cloudy eye" turn into "no eye" without treatment... I strongly suggest you QT this fish and begin antibiotic treatment ASAP.
Furan-2 and/or Kanaplex would be my suggestions (you can combine them).
Erythromycin (i.e. Maracyn) might be easier for you to find, and would be an acceptable substitute. Only combine that with minocycline (i.e. Maracyn 2), however.