Looks like popeye. From the web:
Causes:
Physical injury
Infection
Metabolic disorder
Poor water conditions
Treatment:
Treatment for pop-eye will depend upon the underlying cause. If the eye has been injured, palliative care using aquarium salt unless contraindicated, while the eye heals is advisable. Regular water changes are also recommended, as well as monitoring of water chemistry. If water tests indicate a problem, either with drifting pH or elevated ammonia or nitrites, it should be corrected promptly. Fish should be fed good quality foods to support a healthy immune system.
Fish that are clearly suffering from an infection should be moved to a quarantine tank to avoid infecting other fish. They can also be treated with broad spectrum antibiotics to resolve the infection. If several fish are infected, it may be necessary to treat the main tank with broad spectrum antibiotics. Keep the tank very clean, and perform regular water changes weekly.
Water changes
Use aquarium salt
Correct water chemistry
Good tank maintenance
Broad spectrum antibiotics
Quarantine infectious fish
