Koi Diseases and Their Treatments

One of the most common diseases found in Koi is Ich. It is a protozoan that begins its growth in the pond and later attaches to the gills of the Koi as it matures. Ich can kill smaller Koi, especially in crowded ponds.

An Ich infection shows up as a group of white spots dotted all over the body of the 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 high 70’s F while increasing aeration.  This 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.


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 fish stand a better chance of surviving. Dropsy is curable, even though common understanding of it suggests otherwise. It can be cured with one of many of the anti-bacterial products found in pet stores today.

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 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 Medi-Koi (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.


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. Argulus 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.

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


Some causes of fish infections, such as fungal hyphae or spores, require a microscope to see them. 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. 

Treating Fungus: 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.

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.

Your Koi fish may have  Lymphocystis, also known as Carp Pox if it has any buff discoloration on its skin. Another sign of Lymphocystis is shiny, greasy looking skin. This condition occurs when the water temperature in the fish’s environment has changed. Lymphocystis and Epistylis look alike, but they each respond differently when salt is added to the fish’s environment. Carefully watch the fish for a week after adding salt.  If you still notice the symptoms mentioned above, then the fish is suffering from Epistylis. Epistylis 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. Change the water in the pond and add salt to combat these parasites.


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.

Treatment: 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.  

Treatment: 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 Aqua Prazzi,  potassium permanganate or Fluke Solve (both AquaPrazzi and Fluke Solve have the same active ingredient; “praziquantel”).  


Oodinium parasites can also cause disease in Koi. The disease caused is sometimes called Velvet Disease because it resembles a velvety golden dust, which covers the fish. Adding salt to the pond’s water does not normally clear up Velvet Disease. The best option is to add Formalin to the water as this has been shown to be most effective.


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.  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. Though these parasites can affect the skin they will also infect the gills of your koi as well.  

Treatment: There are treatment options for costia.  Malachite green and formalin can be used but you will need to make sure there is no salt in the pond to start with.  As with flukes, potassium permanganate can be used to treat costia.  Some practitioners will recommend a strong salt bath almost equivalent to that of ocean strength salt concentrations (around 3%).  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 blood (hemolymph) 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.

White Spot 

If you notice that your Koi fish has white spots on its body, it may have a common disease known by the same name, White Spot. The disease can quickly multiply and spread to the other fish in your pond. White Spot is caused by a protozoan parasite that initially appears like little white grains of salt on your Koi. If left untreated, the parasite can cause other bacterial infections in your fish and can even be fatal to Koi. If you notice the parasite on your fish and notice that it later falls off, the pond’s infection is not cleared up. The parasite can reproduce at the bottom of your pond.

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 is caused by a bacterial infection while other times it is due to trauma from running into the side of a concrete pond.  

Treatment: 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 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.

Treatment: Treatments for cloudy eye are a medicated koi food formulated to handle fungal and bacterial issues called Medi-Koi 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.  

Treatment: 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. Maintain a healthy, clean pond and treat ulcers with an antibacterial to prevent any further occurrences.

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.

One of the easiest protozoan parasites to see under a microscope, and therefore confirm your fish is infected, is Trichodina. An infection with 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.

Treamtent: Treat this disease with potassium permanganate. If left untreated, the amount of damage to the fish’s gills can be significant.

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 flicking and flashing and 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.

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.