Are you sure that they faught? it sounds like you may have an ammonia situation going on. Have you noticed any aggression between the fish ? the reason being if the oscar has eye damage the GT has the appearence of being beaten which is what ammonia burns will look like sometimes. & a dead front. so suddenly, check your water parameters. something similar happened to me yrs. ago (diff. fish species) & I thaught it was due ti fighting but it turned out soap was in the pail I used for water change so was inadvertently put in the tank thank goodness it was only small amount so I put the fish in another tank until that one was completly rinsed out.
