You want to start with looking at your water quality. Test the water for ammonia, nitrite, nitrate and PH. Also check your filters, make sure there's a good strong flow coming out of them and there's no waste build up on the filter media itself.
Try doing daily partial water changes for the next several days using a good quality declorinator. The fish may have scraped the eye against something or it may be a bacterial infection in which case the water changes/water quality improvement could start to turn things around. If after 3-4 days you don't see that the eye is improving, or if you see white or gray fuzzy growth anywhere on the fish and/or the fish's behavior changes in any way, (becomes reclusive, stops eating), then you should begin treatment with an antibiotic ASAP.
Robin
Improving water movement with a jet or additional filter may also help the fish recover.