koi diseases


This article describes dozens of koi diseases as well as the most appropriate treatments. The sooner you can diagnose, the sooner you can treat.

Its probably not surprise but for a koi, life can be full of peril. The fish in an outdoor pond are subject to not only birds of prey and raccoon but to things you often can’t see that live in your water and infect your fish.

Fortunately, there is a great deal of information on koi diseases and an equal amount of treatments but the most important thing is being informed on those diseases. The more you know about diseases of koi, the better you can treat them.

Listed below are most of the more common koi diseases and the best treatments.


One of the most common diseases found in Koi is Ich (aka; White Spot disease). It is a protozoan that begins its growth in the pond and later attaches to the gills of the Koi as it matures.

The ich parasite initially appears like little white grains of salt on your Koi. Ich can kill smaller Koi, especially in crowded ponds and can cause other bacterial infections in your fish.  It is usually borne out of poor water quality so to deal with the disease at its root, proper water chemistry will have to be achieved.

Tropical fish specialists frequently deal with Ich. It can, however, also infect cold water fish. They must attach to the fish within 48 hours of hatching or they will die. Once they attach themselves to fish, they dig into the skin and feed on the fish’s tissue.

After feeding on the fish for approximately three weeks, they detach themselves and move to the bottom of the pond to reproduce. The Ich hatch out of cysts at the bottom of the pond and use small hair-like tentacles called cilia to swim about.

Treating Ich

There are a couple of widely used methods to treat ich. The first one requires the koi owner to increase the salt concentration of the pond or quarantine tank to about 0.5% over a period of days. At the same time increase the temperature of the water gradually to the mid 80’s F while increasing aeration. 

This quarantine should last 2 weeks and is an effective and cheaper way to deal with the ich protozoan. The second method is a tried and true method for dealing with parasites in general and involves malachite green and formalin.

Using both simultaneously is definitely going to be effective against ich but you may be able to simply treat with malachite green alone.

For short treatments in quarantine, one could do a quantity of 1.5 mg of malachite green for every liter of water for up to 1 hour (or 6mg of malachite green for every gallon of water). Always wear gloves when handling both malachite green and formalin.

Dropsy (Pinecone Disease)

Fish that have dropsy often show signs of swelling or lifting of the scales. Their eyes will frequently bulge. If you ever notice this, don’t hesitate to separate the infected fish from the others. Paying careful attention to your fish in order to notice these symptoms as early as possible means that your other fish stand a better chance of surviving.

Treating Dropsy

Dropsy is, more or less, terminal. By the time you see your fish bloated with standing scales it means that there is kidney and liver failure.  Prevention of infection to your other fish is recommended with MedFinn or Debride RX. 

Tail Rot

Tail rot and fin rot are really secondary to the original problem which could be stress or poor water quality that is causing your koi to be immuno-compromised in the first place. Once they are weakened then the already-present bacteria can move in a cause physical damage to your koi’s fins.

Treating Tail Rot

One good start to treating this malady is by a 30 -50% water change of your pond. Using Mela-Fix is also a great option for dealing with the bacteria themselves and you can also add salt to your pond which will go to work on your bacteria and also lower stress levels for your koi. If you’d like you can also feed them MedFinn (just as with the cloudy eye treatment).

Mouth Rot

Mouth rot is a symptom of something larger and that is usually poor water quality that is causing your fish’s health to be compromised and open to infection. You’re going to want to stop feeding for the time being and start improving your water quality by doing a 30% water change.

Next thing would be a light salt bath treatment and perhaps a treatment of Mela-Fix. You will also want to monitor your water quality by testing it regularly. You can also remove the fish and treat the sores with hydrogen peroxide or iodine.


Some of the symptoms included with this protozoan parasite are: dying on the surface and rolling over on the fish’s s sides. This is a notorious killer of koi.

Treating Chilodonella 

You will want to increase the salt concentration in the pond for 2 weeks as well as increasing aeration.

Aeromonas Bacteria

These bacteria are associated with ulcers and fin erosion.

Treating Aeromonas 

The koi will need to get injections of Chloramphenicol.  This treatment should yield positive results in 3-4 days.

Pseudomonas bacteria 

Like its cousin, aeromonas, this bacteria is also associated with ulcers and fin erosion.

Treating Pseudomonas 

Infections of this nature will require an injection of Baytril.


This bacteria will attack sites of injury but will cause fin, tail and mouth rot.  Additionally fish can be vulnerable to it during times of stress. 

Your fish may develop a white film on their skin and display sunken in eyes.  It can be a rapid killer so be sure to take swift action.

Treating Columnaris 

Feeding your koi MedFinn or Debride RX will definitely help your fish with an “inside out” treatment strategy. 

Anchor worm (Lernea)

Anchor worm, also known as Lernea is a crustacean parasite that attaches to, and digs into the skin of fish. The female Lernea attaches to fish while males do not. The worm feeds on the fish, damaging its tissue.

This leads to a bacterial and/or fungal infection on the fish. Another crustacean parasite, Argulus also attaches itself to fish and causes damage to tissue.

Treating Anchor Worm 

They can be removed using tweezers or other small pincers. After removal, rub some Neosporin on the infected area. Use Dimilin, Dylox or Lufenuron to treat the pond. 

Though Dimilin is probably your best bet for clearing up anchor worm there are other products out there that have a good track records as well and those include ECORX and Anchors Away.

Argulus (Fish Lice)

These unwanted invaders have eight legs and rounded bodies. They also have to big suckers which are used to attach to the fish. Their appearance leads to their nickname, “fish lice”.

These parasites can cause considerable irritation to fish and can lead to bacterial infections.  The irritation will lead to flashing and rubbing which should tip you off that something is wrong and the koi are trying to get some relief.

Treating Argulus  

As with anchor worm, treatment will consist of Dimilin but another product also works as well, Lufenuron.  You should see positive results in a matter of days.


Mild or moderate infections can be treated and possibly cured if they are caught early. These types of fungal infections are not typically contagious and usually only a single Koi becomes infected.  

Fungal infections in fish almost always begin externally and start through a break in the outer skin layer of the fish. Affected Koi usually display fluffy or cotton-like growths on their skin.

These growths may also exhibit a green tinge because of algae growth on the fungus. There may also be raised white, brown, yellow or green irregular bumps seen on the fish’s fins. Most infections can be successfully treated if caught early so it is important to visually inspect your fish frequently.

Treating Fungus 

If the water temperatures are cold its most likely fungus however epistylis is also a possibility.  Fungus can be removed by gently rubbing the area with a cotton swab while the infected area must be treated with an antibiotic or antimicrobial cream immediately afterwards.  

If you are not sure if its fungus or epistylis try raising the salinity of the pond or quarantine tank- fungus won’t be negatively affected by salt but epistylis will.


Your Koi fish may have Lymphocystis if its displaying any buff discoloration on its skin. Another sign of Lymphocystis is rough, raised lesions on its skin. 

Though not typically contagious or fatal it is can be disfiguring to the fish carrying this virus.  This condition often occurs when the water temperature in the fish’s environment has changed. 

Treating Lymphocystis 

You will want to put the infected fish in a quarantine tank and increase the heat.  There is a treatment containing “neutral acriflavine” that will help defeat the virus (this product takes the brand name “Lymphocystis Cure”). 

It is entirely possible and not uncommon for the virus to go away on its own.


This is an uncommon parasitic infection that is dangerous and can cause other diseases in your Koi. It is usually caused by poor management of the water, resulting in dirty water infected with parasites.

If you do not regularly change the water in the pond, it can easily become a host to these parasites. Epistylis looks like a fungus and thrives in ulcers and wounds on Koi fish.

You can identify an infected fish if you see white colored tufts in and around ulcers and wounds on the fish’s skin.

Treating Epistylis 

Change the water in the pond and add salt (0.3% concentration) to combat these parasites.  One can also put the affected fish in quarantine for 2 weeks while raising salt levels to 0.3%.

Its always nice to have a comprehensive reference manual you can look at when concerned about your koi’s health. My “go to” manuals are the Manual of Koi Health and Koi Health and Disease (Ed. 2). Edition 2 of “Koi Health and Disease” is supposed to be much more comprehensive than the first edition (the one I own) plus it has quick reference charts for disease diagnosis and its spiral bound for ease of use.

Koi Health & Disease: Everything You Need To Know 2nd Edition

Fish normally eat less during the winter and by eating less, fish tend to lose a bit of weight. This normal change sometimes masks Skinny disease.

Skinny Disease

The disease is caused by a bacterial infection that causes the fish to have a sucked-in gill appearance. Its head will often appear much bigger than the rest of its body.

Treating Skinny Disease 

Adding extra food to the fish’s diet can usually clear up this disease. However, sometimes this doesn’t help and if the bacterial infection persists, adding erythromycin to the fish’s food normally clears the infection up quickly.

Carp Pox

Similar in nature to Lymphocystis, carp pox is common, not very contagious, not usually fatal  but can disfigure.  Unlike its cousin, this virus causes soft and waxy looking raised growths. They are often described as being like pinkish, melted wax on the koi’s skin.

Treating Carp Pox 

There is no effective treatment for this virus but fortunately it usually goes away on its own.  Gradually heating the water may expedite the virus diminishing.  For some pond owners this is just something that shows up every winter and spring and goes away with the onset of summer.

Spring Viremia of Carp (SVC)

As the name suggests, this viral disease likes cold water and shows up in spring when the water is still cold and your koi’s immune system is not in full swing.  Some of the symptoms that you may see are a reddening of the skin and swim bladder inflammation. 

It isn’t usually a primary disease as it will normally only come in after another debilitating affliction like a bacterial infection distresses the fish.  Fortunately, SVC is not usually fatal.


This disease also known as “hole in the head” disease is protozoan parasite responsible for lesions in the dermal layers of koi.  You will notice your koi being lethargic and, more definitively, you will notice an erosion of the skin (and usually the fins as well). 

Your koi will isolate themselves in a given area of the pond and, as a result of the disease, will darken in color.

Treating Hexamita 

As with many diseases, you will want to quarantine this fish.  You will also want to establish a treatment course with a product called Flagyl for 10-12 days.

Cotton Wool Disease

Another disease Koi sometimes suffer is called Columnaris, which is sometimes also called Cotton Wool Disease. White threads in the fish’s mouth and a dry skin appearance are the main characteristics of this disease.

Sometimes the color of the Koi becomes darker and white sores can appear on its skin. It is not always easy however to determine if the fish has the fungus in its gills.

However, if the Koi stays near the surface of the pond, gulping for air, it is a good indication that the fungus is, in fact, in the gills. The Koi can also develop a soggy belly and a slimy coating over its skin.

Treating Cotton Wool 

You can put potassium permanganate in the water to help clear up the condition. Additionally, injecting antibiotics and treating the wound directly will help care for the disease.

You should separate the infected Koi from the population and treat the water so the other fish do not become infected.


There are two main classes of flukes; 1) gill flukes and 2) skin flukes. Both kinds are microscopic so if you suspect your fish are suffering from flukes you will need a microscope to verify it.

Dactylogyrus or gill flukes will attach to the gills and, as the name suggests, body flukes or gyrodactylus attach to the body.

Having hitch hikers is usually not a problem…except if you’re a koi and they are parasitic! Flukes tend to eat away at the protective “slime” coating of your fish thereby exposing them to harmful bacterial infections and causing ulcers.

They also are an irritant and cause your fish to itch and subsequently rub themselves on the bottom or side of the pond in an attempt to itch and rid themselves of the parasite.

Gill flukes can eat away at your koi’s gills so much that the koi will not be able to assimilate oxygen from the water. If you suspect a fluke infection be sure to look out for this behavior.

Treating Flukes 

You can quarantine but it is likely that your entire pond is afflicted with the flukes or their eggs so you can treat with medications like potassium permanganate or Aquascape Praziquantel.  Some additional treatments are SupaVerm (will kill goldfish but won’t affect your beneficial bacteria) and Fluke Tabs.

Oodinium Parasites

The disease caused by these parasites is sometimes called Velvet Disease because it resembles a velvety golden dust, which covers the fish.  Other symptoms include ragged fins and a loss of scales and skin.  Though it is somewhat rare there are treatments.

Treating Oodinium

Adding salt to the pond’s water does not normally clear up Velvet Disease. The best option is to add 37% Formalin to the water as this has been shown to be most effective.  Alternatively, Simazine can also be used to treat oodinium.


Costia, like Flukes, are a parasite. In this case it is a microscopic flagellate that can reproduce rapidly. Typically koi don’t suffer from costia unless they are already compromised in some way to begin with so it would be classified as a secondary illness. 

This affliction is usually associated with spring time. Your koi will appear lethargic and will attempt to flash or rub on the sides or bottom of the pond to itch and rid themselves of the parasite.

The skin will also appear whitish/grayish on the infected are of the koi’s body and potentially reddening fins.

Though these parasites can affect the skin they will also infect the gills of your koi as well therefore you may see gasping at the surface.

Treating Costia 

There are treatment options for costia. Malachite green and 37% formalin can be used but you will need to make sure there is no salt in the pond to start with.  If this treatment is chosen be sure to increase aeration.   

As with flukes, potassium permanganate can be used to treat costia. Some practitioners will recommend a strong salt bath (up to 0.6%) while other treatments include trypaflavine which goes by the trade name Acriflavine and copper.

Copper is used because most invertebrates, like costia flagellates, have copper-based hemolymph (blood) and therefore copper as a treatment in ponds is toxic to them.

Acriflavine should be added to your pond at a rate of 1ml/liter and copper is most appropriate at 2 mg/liter. It is critical that you do not overdose with these two treatments.

Pop Eye

Pop eye is also called exophthalmia and is really not linked to any one illness but is more of a symptom of something more. It is a direct response to an excess of fluid or possibly gasses built up behind the eye which then cause the eye to bulge is distend.

There could be several things that could cause this to happen. Sometimes this malady is caused by a bacterial infection while other times it is due to trauma from running into the side of a concrete pond.

Treating Pop Eye 

The best course of action is to immediately quarantine the fish and do a light salt bath. You will also want to greatly reduce feeding and how much you give it. You will also want to do 25% water changes daily and monitor water quality on a regular basis.

Cloudy Eye

Cloudy eye is somewhat uncommon but it can occur. This malady is typically a symptom of something else going on rather than something attacking the eye specifically.

Some causes are bacterial infection, a cataract or even lack of the right kinds of foods (which is why you want to diversify their diet and feed them only quality food with less fillers).

On rare occasions the reason could be a fluke on the eye but sometimes its a simple as physical damage to the eye by running into something in the pond. Both eyes can have pop eye or just one, either is possible.

Treating Cloudy Eye 

Treatments for cloudy eye are a medicated koi food formulated to handle fungal and bacterial issues called MedFinn and or Mela-Fix. Mela-Fix is actually an oil extract from the Melaleuca tree mostly occurring in Australia and it has natural anti-bacterial properties.


Unfortunately, leeches aren’t just looking for a free ride but also a free meal and eventually they will cause mortality of the host fish if left untreated.

Leeches can also transmit the koi disease known as SVC. If you think your koi may be victims of leeches you can actually see them on their bodies as the leeches are not microscopic but other symptoms include a darkening or paling of the color of your koi as well as lethargy.

Treating Leeches 

Some treatments for leeches include a product called Masoten which comes in powder form. Another product is called malathion however many recommend against this treatment as it is an organophosphate insecticide and though it will kill leeches it may also harm your koi in the process.


Ulcers on the skin of your fish result from bacterial infections that form on scales, causing them to become red. The infection causes holes, or ulcers on the exterior of the fish and will eventually result in loss of scales if left untreated.

These ulcers are most often caused by poor quality of the water in the pond coupled with depressed koi immune systems in Spring get attacked by parasites carrying bacteria.  

Ulcer Treatment 

Maintain a healthy, clean pond and treat ulcers with a topical antibacterial such as Panalog to stop the bacteria in their tracks.

Saprolegnia Fungus

One of the most common fungal infections found in Koi fish comes from the Saprolegnia fungus. Spores from the fungus can grow on any part of the fish, including its gills. The fungus first attacks the fish by germinating on dead tissue.

The fungus has thread-like hyphae that release a substance that breaks down the tissue. As the fungal infection grows, these juices begin breaking down and destroying living tissue. 

It often appears like cotton wool and may be stained green from algae.  It almost never attacks a healthy fish –typically the koi will already have some sort of breach like an ulcer.

Treating Saprolegnia

The afflicted fish will need to be isolated in a quarantine tank with the temperature raised to at least 77 degrees F.  Additionally, it will be very beneficial to also get your salt levels up to around 0.3%.


One of the easiest protozoan parasites to see under a microscope, and subsequently confirm your fish is infected, is Trichodina. An infection of this parasite can be detected by a gray-white opaque appearance on the body of infected Koi.

Trichodina is a warm water parasite and can survive in the water for a considerable amount of time without a host.

Visually, they are perfectly round with hundreds of little hooks that look like cilia. It rotates continuously as it moves through mucus, causing damage to the Koi’s tissue.

This parasite attacks both the skin and gills of your Koi. Infected fish also often show symptoms such as flashing, rubbing and lethargy.

Treating Trichodina 

Treat this disease with a five day course of increased salinity (0.5 to 0.6%).   Due to increasing tolerance of some organisms to salt treatments a course of formalin may be necessary.

Koi Herpes Virus (KHV)

This virus is a potent one.  Some of the effects of this disease are sloughing off of the skin which leaves the koi vulnerable to bacterial infection.  Your koi will be lethargic and have sores and lesions on its skin, gills and fins. 

Because some diseases have overlapping symptoms, gill lesions are one of the more definitive indications of KHV.  This virus is contagious and carries and high mortality rate so treatment needs to be swift.

Treating Koi Herpes Virus 

Increase salt concentration to around 0.45% and bring the water temperature up about 1 degree F every hour until it gets to 87 degrees. The fish should remain in this situation for 4 days during which medicated food such as MedFinn should be fed. 

Gill Maggots

Another parasite sometimes infecting Koi is the gill maggot. The parasite most often attacks the Koi’s gills and has a maggot-like egg sac appearance. It is a relatively uncommon parasite sometimes found in Koi ponds but when present, can cause Koi quite a bit of irritation.

You will notice the koi flashing and subsequently the gills become less efficient at absorbing oxygen. The most obvious sign of infection is seeing a Koi gasping for air at the surface of the pond.

Treating Gill Maggots 

The product Lice-Solve has been shown to be effective against gill maggots (as well as argulus) but you will need to increase aeration.  You can treat the whole pond or individual fish in quarantine.

Bent Koi

Pond owners may sometimes notice that one of their koi is taking on a bent appearance.  There may be a few explanations for this:

1) scoliosis from lack of ascorbic acid (Vit. C) in their diet

2) electrical discharge into the water and

3) internal air bladder infection  

Scoliosis can be treated by feeding your koi foods high in Vit. C like yellow bell peppers or kale.  

Electrical discharge into your pond may come from submerged pumps that are damaged or otherwise not operating properly and leaking electricity into the water -these discharges can be easily tested for.  

A swim bladder infection will most likely require antibacterial injections by someone with injection experience.


Clearly there are quite a few little “beasties” out there that can make life miserable for your koi however there are a good deal more treatments these days as opposed to say 100 years ago.

Also, because the treatments are more potent these days there is a good chance that you can deal with a disease or parasite quite rapidly and not lose your prize breeding stock -or even just your favorite koi.

As mentioned previously, the two manuals I recommend having around for quick reference on disease are:

manual of koi health     and        Erik johnson koi health and disease


General FAQ

Why is my koi laying on the bottom?

Usually a stressor such as parasites or poor water quality will prompt koi to lay on the bottom. Another reason could be low dissolved oxygen in the water causing the koi to be lethargic.

Why aren’t my koi eating?

This can be attributed to one of several things. One of them could be stress, another may be from an internal bacterial infection. As temperatures grow colder in winter, koi’s metabolism slow as does their desire for food.

Why is my koi rubbing itself on the side of the pond?

This is called “flashing” and is typically a sign that your koi has some kind of ecoparasite and is trying to rid itself of them.

Why are parts of my koi, like the tail, becoming red?

This is likely fin rot that is ultimately caused by bacteria. Usually these bacteria exist in your pond due to water quality issues.

What are the white spots on my koi?

This could be one of several things. It is commonly ich but could also be cotton wool disease or perhaps the trichodina parasite.

Why is my koi gasping at the surface?

The most common culprit is insufficient dissolved oxygen in your pond. This is easily remedied by adding an air stone at the bottom of your pond.