Koi Diseases, Fish Disease Symptoms, Fish Disease Identification
build a fish pond

You are here: Home > Koi Health & Wellness Center > Koi Diseases

Koi Diseases

As a hobbyist, you should be aware of the common diseases that can affect your Koi, as well as recognize how they show themselves and what you need to treat these diseases. There are more than 100 known diseases and parasites that can infect most species of pond fish and Koi.

Koi Diseases are the result of interaction between a host, a pathogen (a disease causing agent, like a bacteria , parasite, or virus), and the environment. Diseases can be infectious or noninfectious. Infectious diseases are caused by microorganisms that challenge the fish’s defense mechanisms by invading, infecting, and disrupting the normal functioning of the host. Noninfectious diseases are caused by factors such as environmental conditions, nutritional deficiencies, or toxins.

Because fish are cold-blooded animals, their entire metabolism, and therefore their response to infection, depends on the water temperature. The activities of many pathogens present in the pond environment are also linked to water temperature.

In nature, stress is often considered to be a good thing because it stimulates behaviors and physical changes that allow the fish to deal with difficult situations that may arise. When your Fish are confined to a Pond, they are unable to escape the stressor. With prolonged exposure to a stressor this will cause your Koi’s immune system to weaken which makes them vulnerable to disease.

Koi do not jump for happiness, they jump to get more oxygen into their gills, if they have parasites or gill filament damage. Koi will also jump when the pond has too high TDS levels, lots of heavy metals or other irritating materials in the water that make it undesirable to live in. Koi hate copper in their water and will sometimes swim erratically when levels reach toxic range. Even chlorine can burn the sensitive gill filaments and tissues, making it difficult for the Koi to breathe.

The artificial environment of the pond is frequently the source of chronic stress and secondary infections. If a Koi is prone to disease, the source of the problem must be found before the disease can be successfully treated.

Koi produce excess mucus to combat problems. It's like a natural defense they have, so when you see Koi producing excess mucus it's a sure sign your Koi are stressed.

In order to prevent diseases in your Koi, you must provide a stress-free environment (as much as possible) by ensuring proper water quality and feeding a nutritious diet. Regular partial water changes will keep wastes from accumulating in the pond. Frequent water testing is vital in monitoring the status of the pond water. The actual frequency will vary depending on the age of the pond, how well established and regulated it is, the type of filtration provided, and the number of fish in the pond.

Having ruled out poor water quality as the cause of any problems, the next step in determining why your Koi are not behaving as you would expect is to start looking for signs of disease and, if found, to identify exactly which disease it is. Too often people see their Koi hanging in the water, flicking against the bottom, or gasping for air around airstones, and simply dose the water with an "anti-everything" treatment. Although this is better than doing nothing, it can create its own problems for many diseases require specific treatment and this scattergun approach generally will have no positive effect. Before any medications are added, it makes sense to know exactly what you are treating.

Gill infections and ulcers can be indirectly caused by gas super saturation, which occurs when the pressure drops or the temperature of the inflowing water increases significantly.

Dissolved gases, mainly nitrogen, cause small bubbles (gas emboli) to form in the blood vessels of the gills and skin. These bubbles block the flow of blood, damaging surrounding tissue and causing secondary infections when the tissue dies.

How will you know if you have a gas embolism problem?

Stick your hand into the water and if bubbles form on the hairs on the back of your hand when you submerge it, you may have a gas embolism problem.

Poor water quality causes more Koi diseases than any true fish disease. Koi rarely die from a pollutant introduced to the pond, the problem is usually related to filtration or dissolved oxygen.

What is "Aeromonas Alley"?

Koi Fish Diseases

anchor worm

Anchor Worm

Anchor Worm is a tiny thread-like crustacean that buries itself under the scales of Koi and then enters the dermis. Heavy infestations can severely deplete a Koi's strength and lay it open to secondary infection.

You will notice flashing and rubbing as signs your Koi has Anchor Worm.

There is more information about the Anchor Worm in the Koi Pond Guide's Ebook All About Koi. You can also Click Here for more information on Anchor Worm .

Carp Pox Virus

This condition shows itself in the form of gray transparent blister-like spots which increase in size and eventually merge.

There is no known cure but fortunately if the conditions thought to cause the problem are corrected then the condition cleans itself over a number of weeks.

For more information on Carp Pox Click Here.

Gill maggots

Gill maggots are the mature females of the parasitic crustacean Ergasilus. Ergasilus (gill maggots) will appear as grayish black and white parasites several millimeters long infesting the gills.

Heavy infestations can cause severe damage, eroding the gill filaments and allowing secondary infections to develop.

An effective treatment for gill maggots is Microbe Lift Paraxoryne.

Mycobacteria (Fish Tuberculosis)

The Mycobacteria are the smallest organisms that are capable of self-replication. Unlike other bacteria, they lack a cell wall.

A zoonotic potential exists with mycobacteria, and people handling infected fish can get lesions on their hands from contamination of cuts or scrapes. This is known as “fish handler’s disease”.

For more information on Mycobacteria Click Here.


There are many leeches in freshwater habitats (maybe over 250 species) and they range in size from small to large and can easily be seen with the naked eye. These parasites are external, blood-feeding animals. They leave circular wounds on the fish. They suck large quantities of blood from their hosts so they need to be removed quickly. They may also be carriers of blood flagellates which are tiny creatures that produce ‘sleeping sickness’ in Koi which appear listless.

Further, the site of leech wounds will attract bacteria and fungi so one way or another, these are creatures you do not need at all.


These small crustaceans can be embedded in flesh, gills or mouth. Some move freely over the body surface.

Cestode or Tapeworm

Larval tapeworms form cysts on or in the internal organs or in the body cavity. Adults are white and worm-like and are found in the intestines.


Quite an interesting subject to view under the power of a microscope, Trichodina looks like tiny parasols or flying saucers. Good news. Trichodina isn’t as bad as the rest and normally just makes the fish itch. Bad news. It does help introduce bad bacteria into the internals of the fish.

Trichodina thrives in the muck in the bottom of the pond. Thorough pond cleanings should eliminate the threat of this parasite.

For more information onTrichodina Click Here.

Cloudy Eye

Like carp pox, this Koi disease is an unspecific condition in which the eye appears to be covered by an opaque film. It is thought to be bacterial and due to incorrect water conditions, but may also be linked to other identifiable Koi diseases. The addition of pond salt to the water may prevent the condition.

Common Colds

Common colds will show itself by the Koi having a dull color; the eyes may appear cloudy. A Common Cold can be caused by fluctuating water temperatures or by a poor diet that stresses the fish. Stabilize water temperature and feed high protein diets until the Koi recovers thereafter maintain more stable conditions.

Argulus (Fish Lice)

Lice will irritate your fish, with hooks on the legs and the stinger that injects toxins and withdraws fluids. This constant piercing of the skin causes inflammation. However, the biggest threat is, as the fish lice move from one location to another they leave a hole in the protective "slime coat" and skin. These "holes" give the opportunistic bacteria such as Aeromonas or Pseudomonas which are one of the biggest causes of deadly ulcers (body sores) to invade your koi and pond fish.

The best prevenion method of Aeromonas and Pseudomonas is KoiZyme.

For more information on Fish Lice (Argulus) including symptoms click here.

ich white spotWhite Spot, Ich or Ichthyophthirius Multifiliis is one of the most common and fatal diseases of Koi. It is a ciliated protozoan that has a complex life cycle. It has a growth stage in the pond, and a mature stage in the skin and gills of koi. Ich can account for mortality in smaller Koi, especially when your pond is crowded.

Ich is a well-know problem to tropical fish keepers, but it also affects cold water species. White spots are seen dotted about the body and fins-they are tiny ciliates which can swim around to find a host-if they do not, they die within 48 hours. They bury themselves into the dermis where they feed on the bodily cells. They then fall from the fish after about three weeks and reproduce in cysts on the pond bottom. They are most active in warm waters. Repeat treatments are necessary so each batch of cysts is killed as it bursts to release free swimming ciliates (which swim by means of many hair-like growths known as cilia).

Pro-Form C is a Great Product if your Koi have Ich or other Protozoal Parasites.

Fungal Diseases

Fungal Diseases are usually external and most always secondary to a break in the integrity of the epidermis (outer skin layer) and associated mucus coating.

The most common pathogen is Saprolegnia, which is a water mold and not a true fungus, according to mycologists (scientists that study fungi).

Fungal hyphae (filaments) and spores (minute reproductive packets) can be observed in a tissue sample under the microscope. Mild to moderate infections might heal with supportive care. The fungal colony can be gently removed with a cotton swab and the underlying wound treated topically with a disinfectant or antibiotic/antimicrobial cream. They are not highly contagious and generally only a single Koi is affected.

The best control for all fungal infections is good management: good water quality, good nutrition and proper handling.

Koi that are affected might display fluffy or “cottony” growths on the skin (these strands may have a green appearance due to algae growth on the fungus) and/or fins and raised white, brown, yellow or greenish irregular raised lumps or plaques on the skin and/or fins.

Generally, it is only stressed or injured Koi that succumb to fungus attack, so always investigate for any underlying causes (such as damage to the Koi’s skin) if a fungus problem occurs.

Pro-Form C is an effective treatment for Fungal Disease.


One of the most common fungal infections of Koi. The fungal spores will grow anywhere on the koi, including the gills, initially germinating on dead tissue. Their threadlike hyphae release digestives so it can absorb it, as the fungus grows these juices start breaking down living tissue.

Saprolegniasis is a fungal disease of fish and fish eggs most commonly caused by the Saprolegnia species called "water molds." Saprolegnia can grow at temperatures ranging from 32° to 95° F but seem to prefer temperatures of 59° to 86° F. The disease will attack an existing injury on the Koi and spread to healthy tissue. Poor water quality (for example, oxygen, or high ammonia) and high organic loads, including the presence of dead eggs, are often associated with Saprolegnia infections.

Disease Signs

Saprolegniasis is often first noticed by seeing fluffy tufts of cotton like material, colored white to shades of gray and brown, on skin, fins, gills, or eyes of fish or fish eggs. These areas are scraped and mounted on a microscope slide for proper diagnosis. Under a microscope, Saprolegnia appears like branching trees called hyphae.

Saprolegniasis is best prevented by good management practices, such as good water quality and circulation, avoidance of crowding to minimize injury (especially during spawning), and good nutrition.

An effective treatment for fungus infections on your Koi is Chloramine-T.

Koi infected with Chilodonella may display a number of symptoms which will differ depending upon whether the infection is limited to the body or has effected the gills.

Chilodonella kills fast and in great numbers.

For more information on Chilodonella including the symptoms click here.

Common Names: Fin Rot, Gill Rot, Mouth Rot/Cotton Wool and Skin Columnaris.

Flavobacterium columnare (previously known as Flexibacter columnaris) is a bacterium which can cause a number of conditions in Koi. It is generally associated with warmer pond temperatures of 59ºF (15ºC) and above. In warm temperatures it can result in rapid fish losses if not treated quickly. It is important not to confuse columnaris with a fungus infection as often the two can appear very similar, but a different course of treatment is required for each.

Finrot can be the result of a bacterial infection, or as a fungal infection, which rots the fin more evenly and is more likely to produce a white edge. It could be mistaken for fin-nipping and it is essential that you treat promptly because if the rot spreads to the body the Koi will die.

Fish which are badly handled or are kept in unclean conditions are likely to suffer this ailment. If one Koi is spotted with it, check every Koi in the pond.

KHV disease may cause 80–100% mortality in affected populations. Affected fish often remain near the surface, swim lethargically, and may exhibit respiratory distress and/or uncoordinated swimming.

This viral disease spreads rapidly when introduced to the pond and a characteristic symptom is erosion of the gill tissue. Eventually, the gills are exposed to the environment and usually respond by producing more mucus and then by cell proliferation, leading to a condition popularly known as "clubbed gills".

To read more about the Koi Herpes Virus and How to Protect your Koi, Click Here.

Cottonwool Disease

This is sometimes known as mouth fungus. Although associated with the head, this Koi disease can affect any part of the body.

It appears like a grayish film that steadily spreads and can become ulcerated. It is highly contagious and can be transferred by equipment or contact with diseased Koi.

It will rot the mouth parts away so must be treated with urgency once spotted.

Treatment is with Pro-Form C.

Addition of salt to the pond water will greatly reduce the possibility of the disease which is most likely under dirty water conditions or by introduction of diseased Koi which are not subjected to quarantine periods before being placed into the pond.


Minor cuts and abrasions can be treated by painting with mercurochrome or povidone-iodine for 3-5 days. An alternative is to place the Koi in an isolation tank containing a suitable solution that will keep the wound disinfected.


Dropsy is a Internal Bacterial Infection that has invaded one or more of the fish's organs. Fish suffering with this condition have scales which stand away from the body after the manner of pine cones as they open.

Dropsy is an incurable infection of the kidney. For more information on Dropsy and the symptoms click here.

Costia (Ichthyobodo necatrix), is a rapid fish killer. Costia is an external parasite which is active in a vast range of temperatures from 36 degrees to 84°F (2° to 29°C). It is a parasite which can survive both as free-swimming organism or attached to a host.

Click Here for More Information and Symptoms of Costia

Fin Rot

Fin rot can be the result of a bacterial infection (Pseudomonas fluorescens, which causes a ragged rotting of the fin), or as a fungal infection (which rots the fin more evenly and is more likely to produce a white edge). Sometimes, both types of infection are seen together. Infection is commonly brought on by bad water conditions, injury, poor diet, or as a secondary infection in a fish which is already stressed by other disease.

Fin rot starts at the edge of the fins, and destroys more and more tissue until it reaches the fin base. If it does reach the fin base, the fish will never be able to regenerate the lost tissue. At this point, the disease may attack the fish's body directly.

Pro Form PC is an effective treatment for Fin Rot.


Gill Flukes Flukes are tiny, often microscopic, trematode worms and may live in all Koi and not be a problem. However, if the fish becomes ill or stressed, this is when such parasites can multiply and create fatal situations.

Flukes are the Number One cause of Koi ulcer disease. They carry pathogenic bacteria on their haptens and inject the bacteria into the sores they create.

The life span of a gill fluke or body fluke in warm water is 14 to 30 days, however, in cold water, eggs and adults can survive up to seven months. This is why it’s so important for koi collectors to treat their fish every spring and fall for flukes.

For more information on

Flukes and it's symptoms Click Here.

Epistylis Picture

Epistylis looks bell-shaped with a long “handle” connected to it. Tiny hair-like cilia on the end of the bell shape may be seen. Cilia are used for controlling movement, and in the case of Epistylis are also used to feed on waterborne bacteria. It may also be seen in its contracted form and in this instance it will simply look circular.


To Learn More About Epistylis Click Here

Black Spot

Typically a "disease" that is most commonly associated with Tangs and Surgeonfishes, but one that can be contracted by other species as well, what is referred to as Black Spot Disease, Tang Disease or Black Ich is actually caused by an infestation of tiny Genus Paravortex member turbellarian flatworms.

Although parasitic organisms that are much less dangerous and life-threatening, as well as rather easy to get rid of compared to other ich diseases such as Oodinium, Cryptocaryon, and Brooklynella, nonetheless it is a problem that needs to be treated upon recognition to eradicate infected fishes of these parasites.

It is caused by larval flukes encysting under the skin or in the flesh.

An effective treatment for Black Spot Disease is Microbe-Lift BSDT

Gas Bubble Disease

"Gas bubble" disease can result when the water is supersaturated with gas (bottom of a water falls or overactive aquarium aerator). The gas released from the supersaturated state in the blood stream of the fish can form gas bubbles which subsequently block circulation.

A specialized structure is the gas (swim) bladder - specialized for buoyancy adjustment, it may be connected to the esophagus. Also, it may have a "gas" gland which enables release or resorption of gas from blood to and from the gas bladder.

Fish Parasites Click Here

Bacterial Diseases Click Here

You are here: Home > Koi Health & Wellness Center > Koi Diseases


The Only Koi Book You Will Ever Need!

Whether you are a beginner or a Koi pond specialist this ebook contains all you need to know to have a beautiful, optimal ecosystem, with healthy pond fish!

~Price $20.00

Pay with Safe and Secure Pay Pal.


After Check out you will be taken to a page to download your Ebook.

Koi Body

  • Barbel These act like Koi taste buds.
  • Brain Set just above and behind the eyes, which provide Koi with excellent vision, the brain runs all bodily processes.
  • Gills Through these, carbon dioxide is releasd into the water and oxygen is collected by red blood cells. The gill tissue sheds ammonia.
  • Heart The three chambers (atrium, ventricle and sinus venosus) pump deoxygenated blood to the gills.
  • Liver This large organ stores food for distribution to the tissues. It breaks down unwanted proteins into ammonia, forms bile from old or nonfunctioning red blood cells and breaks down toxins and poisons.
  • Spleen This stores immature red blood cells and produces cells of the immune system.
  • Intestine Food is digested here (Koi have no stomach), by enzymes, and nourishes the blood supply.
  • Swimbladder Sound waves are processed here, then linked to the inner ear, emabling the Koi to hear.
  • Gonads These sex glands are situated on either side of the body.
  • Vent Urine, excrement, eggs and sperm (milt) are all released here.

For more on Fish anatomy click here.

First Aid Kit For Koi

  • Non iodized Salt
  • Cotton Balls
  • Hydrogen Peroxide
  • Rubber gloves
  • Antibiotic ointment
  • Sedating agents
  • Malachite green and Formalin
  • Potassium permanganate
  • Acriflavine
  • Propolis or a similar wound sealer
  • Sharp pair of scissors
  • Scalpel
  • Good net and bowl in which to catch and handle the Koi.

These few items alone will help you combat a wide range of fish diseases.


Injection of antibiotics gives the best success rate, especially when dealing with advanced bacterial infections. Injections give a measured amount of drug directly into the body, leading to high blood and tissue levels of antibiotic. The disadvantages are that handling will often cause stress and a certain amount of skill is necessary in handling fish during this procedure. In most cases the fish will need to be sedated, especially if there are any lesions that need cleaning and dressing.

If your not comfortable giving your Pond fish and injection then feed Debride RX Medicated Koi Food.

Enjoy This Site?
Then why not use the button below, to add us to your favorite bookmarking service?

Build A Koi Pond Blog
RSS Feeds


Koi Diseases Beginning

Contact Us

PayPal Acceptance Mark

Return to top
Copyright© 2008-2030.Koi-Pond-Guide.com