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When a fish has lost an eye it's as a result of either an illness called Pop Eye, a fight with another fish or an accident in the aquarium.
If the fish has in fact lost an eye to Pop Eye then to prevent further infection it would be advisable to treat the fish with a medicine such as Maracyn.
Also check to see if there are any aggression issues in the fish tank and if there are any sharp objects in the tank that might cause a fish to lose its eye.