Diseases that affect the eyes of pet fish are quite common. Diseased eyes may appear cloudy, exophthalmic (popped out), hemorrhagic (bloody), microphthalmic (small), or even missing. Common causes of eye disease in pet fish include trauma, infection, gas bubble disease (the "bends"), parasites and neoplasia (cancer).
The fish eye is a very interesting organ and is quite similar to the eyes of humans and other mammals. For most teleosts (bony fishes) that live in shallow water with good light penetration, the eye is quite large, and is a prominent and important sense organ. Fish eyes do display some differences to the mammalian eye. First of all, fish don't have eyelids, although some species have specialized tissue that may act like an eyelid. Secondly, the fish lens is inelastic, and does not bend like our lens does in order to focus light onto the retina (one reason why people over 40 need reading glasses is because the human lens becomes less elastic and more like a fish lens over time). Fish accommodate for their "stiff" lens by utilizing special muscles which move the lens in and out, allowing for focused light to reach the retina.
Ocular (eye) disease is common in captive fish because of the eye anatomy and certain environmental factors. It's quite simple really. Big eyes without protective eyelids are vulnerable. Combine this factor with an unnatural habitat (fake coral, large rocks, metal thermometers and plastic SCUBA divers) and aggressive tankmates, and you have the ingredients for traumatic eye disease. Once an eye has been damaged, secondary pathogens such as viruses, bacteria, fungi, and parasites can colonize the eye and make matters much worse. The following is a list of common eye problems and how to treat or prevent them:
Corneal ulcers: The cornea is the clear, protective covering of the eye. Humans have a similar structure. When this thin membrane is scraped, scratched or cut, an erosion or ulcer may develop. In some situations this condition can get progressively worse and in severe cases the eye can even rupture. A veterinarian can diagnose this problem by applying a special stain to the cornea. If the cornea takes up the stain, there is likely an ulcer. In most cases the treatment of choice is antibiotics applied directly to the eye. Some cases may resolve without treatment. Something as simple as a scrape from a net or a nip from a tankmate can cause a corneal ulcer.
Corneal edema: When the cornea becomes irritated or inflamed, it may accumulate excessive fluid (edema), causing it to look cloudy or slightly opaque. In some cases a bacterial infection will contribute to this problem. Your veterinarian can diagnose this condition and rule out more serious eye disorders. With topical antibiotics and good supportive care, fish with this condition generally have a good prognosis.
Cataracts: A cataract is a lens that has become opaque and does not transmit light efficiently. While not common, this condition does occur in fish. Causes include nutritional deficiency, trauma and hereditary factors. Providing a balanced diet with food that is fresh (commercial food less than 6 months old) can help prevent cataracts.
Exophthalmia: Also known as "pop-eye" disease, this condition is more of a syndrome than a specific disease. This condition may be unilateral (one-side) or bilateral (both eyes affected). Causes include systemic infection, poor water quality, supersaturation (gas bubble disease) and trauma. Your veterinarian can help with a diagnosis and treatment plan. Unfortunately, if the eyes are severely affected, the fish may be permanently vision-impaired or even blind.
Missing globe: In some cases, the eye (globe) is so severely damaged that it may fall out or be reabsorbed by the fish. Fish have an amazing ability to heal, and as long as there are no underlying disease problems, they will frequently recover from a lost eye. Once the eye itself is gone, scar tissue will quickly fill the orbit (socket), and within a matter of several weeks, the orbit will be filled with new tissue. While this may not be cosmetically pleasing, many fish do quite well with one functional eye.
The best way to avoid eye problems in your pet fish is to learn as much as possible about the natural history and proper husbandry of your pet. This includes knowledge about nutrition, aquarium or pond maintenance, fish handling and tankmate aggression. If you notice an eye problem in your fish, contact a veterinarian or fish health specialist as soon as possible so the problem can be identified and managed effectively.