Koi Fish Eyes - How they Work and How to Keep Healthy

Poor Water Conditions May Lead to Infections Affecting the Eyes of Your Koi

Koi rely quite heavily on their eyes while in your pond. They use their eyes in conjunction with their barbels to find food and to sense potential danger in the water. It is vital that you're able to provide an environment that protects the vision of your koi.

Have you noticed that your koi fish eyes look cloudy, are abnormally protruding, or otherwise do not look quite right? If so, you are undoubtedly concerned that your koi fish may be sick. Have you tried to locate information on koi fish eye conditions but became overwhelmed by the sheer amount of information available?

A Red and White Koi Fish with a Slight Popeye
An Eye Condition May Occur Due To Various Underlying Diseases

If so, fortunately, you are in the right place. This article provides you with information on koi fish eye diseases to help you identify, properly treat, and potentially prevent koi eye problems.

Normal Functioning of Koi Fish Eyes

In humans, light passes through the clear front layer of the eye (cornea), which bends light, enabling the eye to focus. The colored portion of the eye (iris) controls how much light is allowed in the eye through the opening called the pupil. The light then passes through a clear inner portion of the eye (lens) which, together with the cornea, properly focuses light on the retina.

The human retina is the thin nerve layer that lines the back of the eye. It receives the focused light, converts it into electrical nerve signals, and sends the signals via the optic nerve to the brain for visual recognition.

A Close-up Photo of a Koi Eye
Koi's Eye Conditions Might Deteriorate Over Time and Lead to Vision Loss.

Although koi fish eyes are similar to that of humans and other mammals, there are some differences, including the following:

  • Koi fish eyes have lenses that tend to be more spherical.
  • The koi eye lens is attached by a muscle and ligament to the back of the eye, allowing them to move the lens closer to or further from the retina. This enables them to bend light and focus it on the retina.
  • As with human eyes, koi fish eyes have rods and cones and can process black-and-white and color images. However, the color spectrum and light intensity processed by koi differ from that of humans.

Looking into koi fish eyes can be a helpful window into their health and well-being.

Recognizing Koi Fish Eye Diseases

Healthy koi fish have eyes that are spherical with a dark, clear lens. If the shape, structure, color, or luster appears abnormal, various health conditions or diseases may be responsible. For example, you may detect the following koi fish eye abnormalities:

A Yellow Koi Hiding Behind Lily Pads
Recognizing Eye Diseases Can Help Prevent Vision Loss
  • In Popeye disease (exophthalmia), one or both eyes swell and protrude from the eye socket.
  • Cloudiness of the eye
  • Poor eye luster due to poor nutrition and lack of vitamins
  • Deformity of the eye
  • Bleeding of the eye

Popeye Disease and Dropsy

In koi fish with Popeye disease, leakage of fluid behind the eyeball may cause swelling. The eye may appear cloudy, discolored, or even bloodstained (if caused by an injury). In serious cases, infected eyes affected by Popeye disease may rupture if treatment is not received. The koi may recover over time but will experience blindness of the ruptured eye.

Potential Causes of Popeye Disease in Koi Fish

There are several potential causes of Popeye disease, including the following:

  • If the Popeye disease is unilateral - particularly if just one koi fish is affected - the cause was probably an injury. It is possible that the koi bumped or scraped its eye against something in the pond or aquarium or got into a fight with another fish. 

Examine the eye to determine whether there is any damage, which will strongly suggest that an injury caused the swelling. In most cases, as the injured eye heals, the swelling eases and the protruding eye begins to recede. Yet be sure to monitor your fish carefully since a damaged eye may become infected, which in turn could cause vision loss.

A White and Orange Koi with Dropsy
Regularly Monitor the Pond's Temperature to Prevent Koi Stress
  • If the Popeye disease is bilateral, the underlying cause may be an infection due to parasites, viruses, bacteria, or fungi. If your koi are affected by popeye and dropsy, i.e. swelling (edema) of the belly, it’s difficult to treat. 

Also known as Pinecone or Bloater disease, dropsy causes the eyes to protrude and the fish scales to stick out from the body. Pressure secondary to fluid build-up causes this abnormal pinecone appearance.

In addition to popeye, a distended belly, and raised scales, symptoms of dropsy may include loss of appetite, gasping, and unbalanced swimming.

Such symptoms usually develop in the final stages of bacterial infection, where there is often widespread internal organ damage and failure. As a result, dropsy unfortunately has a high mortality rate. Such bacterial infection may be caused by Pseudomonas or Aeromonas bacteria, the most common bacteria found in koi ponds. These bacteria can enter the body through wounds or open sores on the fish caused by parasite bites.

A Group of Koi with Various Colors and Patterns
A Photo of Healthy Koi Swimming

Dropsy may also result from kidney (renal) failure, fluid retention, congenital heart disease, or, in rare cases, viral infection or Mitraspora cyprini, a rare parasitic infection that attacks the kidneys.

Treatment of Popeye Disease or Dropsy in Koi Fish

The treatment for koi fish with Popeye disease will vary depending on the cause. In cases where popeye develops due to an injury, relieve pain by using aquarium salt until the injury heals, unless it is specifically contraindicated for your koi.

As your koi is recovering, regularly change the water and carefully monitor the water’s chemistry. If your water tests show high levels of ammonia or nitrite or improper pH level (pH drift), immediately take necessary measures to correct the issue to prevent further stress on your koi.

An Orange Koi with a Bacterial Infection
Monitor Your Koi To Avoid Bacterial Infections

If your koi has a bacterial infection, move it to a quarantined fish tank to prevent the infection from spreading to your other koi. Ask your aquatic veterinarian or pet supplier to recommend an appropriate broad-spectrum antibiotic fish food to treat the bacterial infection or your aquarium and pond water if more than one koi has the bacterial infection.

If you suspect that your koi has a parasitic infection, your aquatic veterinarian can conduct skin or gill biopsies to confirm the diagnosis and provide appropriate treatment.

Cloudy Eye Disease in Koi Fish

Koi fish may develop slight clouding over the eyes or, in some cases, the eyes may become a nearly opaque white. But rather than a disease, cloudy eye is a condition that develops secondary to an underlying cause, such as parasitic or bacterial infection. Because this may be a symptom of a serious underlying condition, it is crucial to identify and treat its cause immediately.

A Red Koi Fish With Its Mouth Open
Antibiotic Food May Be Needed To Resolve an Eye Infection

Symptoms of Cloudy Eyes in Koi Fish

The symptoms associated with cloudy eyes may be variable depending on the underlying cause. However, they may include the following:

  • Koi may have a cloudy, potentially opaque film over the surface of the cornea that may extend deeper in severe cases
  • Because affected koi are unable to see well and have reduced depth perception, they may demonstrate unusual swimming behaviors and have difficulty feeding.
  • If cloudy eyes are due to degraded water quality and infection, you may note that your koi is rubbing itself against plants, rocks, and walls and has reddish or pinkish skin irritation.
  • If koi have an infection, they may produce excessive yellowish mucus.
  • If cloudy eyes are due to malnutrition or infections, your koi may become lethargic, begin to lose weight, and have damaged or dull scales.
An Orange Infected Koi with Bumps on its Head
It is Best To Contact Your Aquatic Vetenerarian for Any Signs of Infection

Potential Causes of Cloudy Eyes in Koi Fish

Cloudy eyes may develop due to low water pH, poor water quality, improper use of water treatments, injury, bacteria, viruses, parasites, or fungi. The most frequent causes are bacteria and degraded water quality.

Koi cloudy eyes may develop due to the following:

  • Skin or eye flukes: worm-like parasites with hooks that they use to attach themselves to the fish. Affected koi tend to rub against objects and jump out of the water due to irritation.
  • Ich (also known as white spot disease): a disease caused by a parasite (ichthyophthirius multifiliis) that burrows and becomes encysted under fish skin, leading to the development of white specks across the fins, scales, and gills
  • Dietary deficiencies or malnutrition
  • Cataracts
  • Bacterial or fungal infections in wounds
Two Dead White Fish Due To Poor Water Quality
Vitamin A Deficiency, For Example, May Contribute to Poor Fish Eye Health

Poor water quality may often be responsible if your koi fish develops cloudy eyes due to fungal, parasitic, or bacterial infection. However, in some cases, introducing new koi may cause said infection. Therefore, it’s always a good idea to quarantine any new fish for several days to detect infections that could spread to your other koi.

If a koi has just one cloudy eye, chances are that it injured the eye. Or if your koi is an older fish, the cloudy eye may be due to cataracts.

If a koi has bilateral cloudy eyes and other fish also share the same condition, this suggests a bacterial or other infection that should be diagnosed and treated as promptly as possible. Contact your aquatic veterinarian, describe the situation, and ask whether you should take water samples from the pond and light skin scrapings from your fish for evaluation.

Treatment of Cloudy Eyes in Koi Fish

Again, the treatment of cloudy eyes may vary depending upon the underlying cause. Such steps may include the following:

A Dead Yellow Koi in a Net
It is Important to Provide Supportive Care to Your Fish
  • Provide your koi with a healthy diet that contains sufficient nutrients, with approximately 35 to 40 percent plant- or meat-based proteins, low carbohydrates, and an appropriate balance of minerals and vitamins.
  • If your koi fish begins to develop cloudy eye(s), immediately test water quality for the proper levels of ammonia, nitrites, chlorine, and pH and sufficient dissolved oxygen levels. Take necessary steps to correct any imbalances. Also, be sure to install an aerator and filter in your pond or aquarium, and conduct small water changes daily or up to 20 percent water changes every week.
  • Add various plants to your pond or aquarium, including floating and submerged plants. Such vegetation will help to regulate water temperature, filter the water naturally, oxygenate the water, and provide an interesting habitat for your koi to explore.
  • If you suspect that infection is the cause of cloudy eyes, contact your aquatic veterinarian or pet supplier to ask about an appropriate broad-spectrum treatment against various parasites, bacterial and fungal infections, and flukes.
  • If treating your pond or aquarium with a broad-spectrum treatment isn’t effective after about a week, ask your aquatic vet or pet supplier about using potassium permanganate, which works against parasites, bacteria, and fungi chemically without seriously damaging your koi’s protective slime coating or skin. It’s also known to be effective against Ich, which often is an underlying cause of cloudy eyes.

Frank Salvatore

In 2015 our family moved into a house south of Denver, Colorado with a  koi pond.  Since that time I've learned to really enjoy the koi fish and the pond. This blog is dedicated to providing helpful hints and information for koi pond hobbyists - as well as those of you who just inherited a koi pond and are thinking NOW WHAT?

About Me

Hey there - I'm Frank Salvatore. In 2015 our family moved into a house south of Denver, Colorado that had a Koi pond. The problem was I knew absolutely NOTHING about koi ponds.

This blog is dedicated to providing helpful hints and information for koi pond hobbyists - as well as those of you who just inherited a koi pond and are thinking NOW WHAT?
Learn More About Me

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