Q. I recently purchased a tiger oscar at a local pet store. Upon bringing my new oscar home, I noticed his eyes had something wrong with them. A strange orange bubble covers the pupil of the left eye. This appears to have caused loss of sight in this eye. A cloudy dot is evident on the surface of the right eye, but the fish still seems to have vision in it. I have seen oscars in other shops affected by this problem, but I have no idea what causes it or how to treat it. Can you help me?

A. The bubble-like structure you describe in your fish's left eye is probably the result of damage inflicted during the course of a fight. The eyes are vulnerable structures, which may explain why so many fish species target them in serious bouts of aggression. Based on my experience with similar injuries, I would suggest that your oscar can still perceive light and movement through the damaged eye, but resolution of detail is no longer possible. Regrettably, such eye damage is irreversible.

Cichlids with such injuries would be poor candidates for life insurance in nature, where predators lurk, but appear to manage very well in captivity. While clearly not a show-quality specimen, your oscar should still make a satisfactory pet and could even be used as breeding stock if care were taken to find a compatible partner.

Milky or cloudy patches on the surface of the eye are usually the result of environmental trauma. Damage due to net abrasion, pH shock or poor water quality usually accounts for this condition. Once the source of stress is removed, the eye should recover its customary clarity rapidly. Supportive treatment in the form of a vitamin B12-based tonic, such as Tetra's African Vital, can do no harm and seems to accelerate regeneration. Such mechanical damage carries with it a risk of bacterial infection. In fish so afflicted, the cloudiness will cover the entire surface of the eye, whose surface may develop a flannel-like appearance. Any furan-based antibiotic used according to the manufacturer's instructions will treat such "eye fungus" effectively.