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Eye problems

Most hobbyists know that once you have made eye contact with a big, tame Koi, you are hooked on Koi for life. The emotions triggered by that fishy stare are far from the emotions that you experience when cage-diving with sharks! Although the eyes of a Koi have a kind of hypnotic charm, it is also the eyes that that can tell an owner a lot about the well being of the fish.

 The eyes are relatively small in relation to the body so it needs careful scrutiny to determine if something is wrong or not. Feeding time is normally the best; otherwise one should bowl a fish to see the eye clearly. The eye construction is very similar to that of other animals. But the lens is virtually spherical to facilitate better sight under the water. The eyes are placed relatively low on the fish's head, which facilitates bottom feeding and almost 360 degree vision. Only a small portion of the eye is visible from the outside. This is usually small and black, but with a fair amount of mobility of the ball within a non-closing circular lid. For fish, Koi have very good sight. The eyesight is however by human standard short-sighted. The vision is clear for plus minus two meters, blurred for the next 10 meters and beyond that a Koi cannot see at all. Koi can distinguish colours but can better identify objects that are green or yellow. The iris cannot contract and dilate like those of land animals.

During the last couple of months, I have witnessed various symptoms and two recent incidents prompted me to share the possible optical diseases or other ailments that the hobbyist can detect by scrutinizing the eyes of a Koi, with readers. I must haste to point out that some ocular diseases are not treatable, while others are merely a manifestation of other problems with the fish or water quality, therefore not a disease of the eye at all. It must further be pointed out that Koi are equipped with amazing senses and even with the total loss of sight, it can lead a normal life with a little bit of care by the hobbyist. The most important is the requirement that it should not be expected to compete vigorously for food.

There are mainly five symptoms that the hobbyist may encounter from time to time that will be an indication of some ailment in a specific fish, therefore looking into the eyes of your collection during feeding time should form an integral part of the visual inspection of your Koi. When eye problems occur, it normally affect one or just a few Koi in the collection and in some cases only one eye, so careful observation is necessary.

Healthy eye

In the healthy eye, the pupil is a uniform black circle, surrounded by a paler outer ring. The colour of this paler outer ring, the iris, can vary considerably depending on the variety and it can even vary between the two eyes of the same fish. It all depends on the markings or background colours.  It is therefore an advantage to know your individual fish, as abnormalities will be spotted more readily.



Eye-worm/eye fluke

This parasite (Diplostomum spathaceum) is not encountered very often by the hobbyist and therefore described in few Koi health books. The reason for this is the complex life cycle of these parasites. Diplostomum spathaceum utilizes many species of fishes as a second intermediate host. The life cycle of eye worm begins as an adult trematode in the intestine of fish eating birds. The body of the adult trematode is 0.3 to 0.5 mm in length.  Eggs are shed and passed through the bird’s feces into the water. These eggs hatch in about 21 days at summer water temperatures into free-swimming ciliated miracidia that seek aquatic snails for a first intermediate host. The miracidia penetrate the snail and metamorphose to a mother sporocyst, then to one or more daughter sporocysts. Each produces many cercariae which are released into the water. The free-swimming cercariae seek second intermediate hosts, which in this case, will be Koi.


Some cercariae which enter the skin, fins, and gills enter the blood stream and are carried to the eyes within 30 minutes of the time of penetration. There are also evidence that transmission is possible by fishes feeding on snails containing cercariae. The eye worm then causes blindness. There is no cure for eye-worm, but it is rarely fatal.


Injury to an eye happens frequently because the eye protection of a Koi is limited. Young fish is particularly excitable and may injure itself against the side of the pond when frightened. Other possible causes are protruding rocks and pipe work in the pond, potted plants or unfortunate accidents during feeding time.

An injured eye my cause some initial confusion and all kinds of thoughts may cross the owners mind.  When an abnormality is spotted, it is worthwhile to take a close look at the eye. Mostly only one eye is affected. The eye may bulge, or it may be just bloodshot. Sometimes it is bloodshot, red and bulging, but it still remains the symptoms of an injury. It all depends on the seriousness of the injury. Severe injury where the eye itself is ripped open can be treated for secondary infection, but the fish will most probably lose its sight in that specific eye.

                    injury              injury_3  
                  injury_4                pop

The treatment for injuries depends on the seriousness of the case and also if it is spotted early or whether infection has already set in. Harsh medication on injured eyes may prevent possible infection, but it may also cause the fish to go blind. Antibiotic ointment, drops or injection and an appropriate antibacterial treatment of the water are the best way to treat an eye injury. Sometimes careful scrutiny with a magnifying glass may reveal that the injury is superficial with no broken skin, and no red swelling caused by infection. There may be blood in the eye and under the surrounding skin. In such cases it is better not to treat the fish and to let it remain in the main pond. Just keep a close watch on it for a few days. I may recover without any interference from the hobbyist.

Parasitic infection

Although not an eye infection as such, a fish’s reaction to a parasitic infection is normally increased production of the protective slime coat. In severe parasitic infection the eyes may also appear clouded over.


When the whole body of a fish is turning grey-white, the eyes are clouded over and it is hanging listlessly close to the surface or waterfall, it is an advanced state of parasitic infection and the prognosis is very poor. Antiparasitic treatment should start immediately. I would not recommend treatment in such a case with Potassium Permanganate or a Formalin-based medication, because in such cased the chances are good that the gill may already be damaged. Salt at 0.3 percent and Seponver Plus may be more appropriate.

Bacterial infection

Cloudy eye can also be caused by a bacterial infection. In such cases, only a part of the eye, or the entire surface or lens of the eye takes on a cloudy, opaque appearance. Bacterial infection of the eye can originate internally or from the outside. If the origin of the infection is internal, the cloudiness will appear to move from deep inside, to the surface of the eye. In cases of a more widespread internal infection, the eyes may be protruding or bulging or have a grayish film over the eyes and protruding. The reason for this is the bacterial damage to other organs like kidney and liver, causing interruption of the osmoregular process. 

If there is a build-up of mucus on the outer surface of the eye, one of the probable causes may also be a bacterial infection. External bacterial infection can manifest itself through the following symptoms:

Translucent film over the lens of the eye
Clear to translucent film over the lens of the eye and part of the body
The iris becomes cloudy
Swelling and redness around the outside of the eye

    cloudy_eye  cloudy_pop   iris_cloudy

Treatment for internal bacterial infection is either medicated food or injections. Please note that an injection against Gram Negative Bacterial infection should be administered. Treatment against external bacterial infection is much less complicated. A series of Salt or Potassium dips are normally sufficient to clear the symptoms.
It may however also be due to problems with the water quality, or a combination of both bacterial infection and poor water quality.

Poor water conditions

Where the entire surface or lens of the eye takes on a cloudy, opaque appearance, it is most commonly caused by poor water conditions. Like in all cases of investigating the cause of a problem, the first step will be to conduct tests on the water quality. Excess mucus or a film over the eyes will develop in cases of unacceptable levels of ammonia, nitrite and nitrate. Improving water conditions usually cures cloudy eyes.

                      cloudy_eye          cloudy_pop

Monitor the fish closely for the following week, because the poor water conditions may have triggered a bacterial/parasitic infection.

Poor nutrition

Inferior quality diets can also result in eye problems Diets with inadequate levels of one or more of the under mentioned vitamins, may cause eye cloudiness or even cataracts.


The vitamins referred to are vitamins A, B2 and C. This kind of ailment is extremely unlikely if a reputable brand of Koi food is used. Several Koi keepers however use a mix of various brands to feed their collection. If there is a deficiency in one brand, it is hoped that the second brand may supplement any possible inadequacy. Reputable Koi food manufacturers however invest a lot of resources to insure a balanced product.


Cataract refers to opacity in the lens of the eye. Cataracts can be due to vitamin deficiency, adverse water temperatures, infection or old age. There is no treatment for a cataract.


Adverse water temperatures

Rapid fluctuations in water temperature may affect the protein structure of the eye lens, causing the development of a cataract. In the average Koi pond this severe fluctuations will be unlikely and if it does appear the Koi will in all probability succumb to some other ailment before the cataract can influence the quality of life of a fish.


Fungal infections

Fungus manifests itself as white to grey growths resembling cotton wool. In time the colour may change to green or brown as algae and other suspended solids get trapped in the fungal growth. Various fungi are present in the koi pond and are generally harmless. If the eyes or any other body surface are damaged through whatever reason, the fungus will attack that specific area.


The visible part of the fungus may look bad, but the main problem with fungus is the damage that it causes on or below the surface of the skin. As with all eye problems, once the symptoms are noticed, the treatment should start promptly to limit the damage to the eye. In this case, a salt-, Potassium Permanganate- or Malachite Green bath/dip is the recommended treatment.


Sometimes a substantial bacterial ulcer may cause water to penetrate the fish by means of osmosis at a faster rate than the kidneys can excrete it, causing exophthalmia or protruding eyes. The effect of the osmosis can be reduced by the addition of salt to the pond water. The following photos of the same fish clearly demonstrate the effect of an ulcer where water penetration causes the eyes to bulge from the internal pressure caused by the excess fluid. There is no need to treat the eyes in this case. The remedy lies in restoring the osmotic balance by the addition of salt to the pond water, and to try and seal the wound with propolis or similar wound treatment.

  osmosis_1        osmosis_2

Last Updated on Sunday, 08 August 2010 12:14