A Guide To Disease In Koi

Koi carp can fall prey to a variety of diseases. Your Koi rely on you to provide a healthy environment in their pond and to ensure that their lives are stress free. Diseases in koi usually result from the following issues:

  • High Ammonia Levels In The Water
  • Low Oxygen Levels
  • Handling The Fish
  • Poor Water Quality
  • Overcrowding In The Pond
  • Parasites
  • Incorrect Water Temperature
  • Toxic Chemicals
  • Sharp Edges In And Around The Pond
  • Inadequate Or Incorrect Nutrition

It is essential that you identify the signs of disease at the earliest opportunity and then take the appropriate steps to tackle any conditions that you discover. Here are the diseases that you should look out.


This is one of the most common diseases found in Koi and is also known as White Spot Disease. The first symptoms of Ich are the appearance of small white parasites on your fish. These look like grains of salt. Ich is usually the result of poor water quality and so you will have to address both the disease and its causes.

To treat Ich you should first increase the salt concentration of your pond to roughly 0.5% over a period of a few days. The water temperature should be increased to 80°F and aeration should be improved. Alternatively you can address Ich using Malachite Green and Formalin. Both treatments are best carried out using a quarantine tank.


This is also known as Pinecone Disease and is generally the result of poor water quality. Koi carp suffering from Dropsy often exhibit swelling or lifting of the scales. Their eyes may also bulge. Again it is best to quarantine any affected fish although the condition is not highly contagious. Dropsy is almost always fatal once the symptoms are visible as they are indicative of kidney and liver failure.

Tail Rot

Tail rot and fin rot are secondary conditions caused by stress or poor water quality as either can compromise the auto-immune system. This is a bacterial infection which is best treated by changing the water in your pond and increasing the levels of salt. Necrotic tissue may need to be trimmed under sedation and severe cases can be treated with antibiotics.

Mouth Rot

This condition causes sores in the mouth and is often caused by poor water quality. You should address the cause by improving the environment in the pond. The sores can be treated with hydrogen peroxide or iodine.


This condition is a killer and is caused by a leaf shaped protozoan parasite. Fish may roll over on their sides and exhibit laboured breathing and lethargy. They may also try to rub against an object in the pond. Treatments include formaldehyde used as a bath, Methylene Blue as well as the drug Acriflavine.

Aeromonas And Pseudomonas Bacterial Infections

These bacteria can cause ulcers and fin erosion. The infected fish will require Chloramphenicol injections for Aeromonas infections and Baytril injections for Pseudomonas infections.


The Columnaris bacteria will attack wounds causing fin, tail and mouth rot. Fish can also develop a white film on their skin and exhibit sunken in eyes. This infection is also known as Cotton Wool Disease and can quickly prove fatal. It can be treated via a bath in Mebromin, Potassium Permanganate, or Methylene Blue.

Anchor Worm

Anchor worm is also known as Lernea and is a crustacean parasite that attaches to koi and feeds causing damage to the tissues of the fish. The worms can be removed using tweezers. Neosporin should then be applied to the infected area. Dimilin, Dylox or Lufenuron can be used to clear the pond of Anchor worm.


Argulus or fish lice are parasites which cause irritation and which can lead to bacterial infections. The irritation will cause the Koi to rub themselves against objects to gain relief. Treatment is as per Anchor worm.

Fungal Infections

This type of infection is not contagious and usually begins with a break to the skin. Fluffy growths will appear which may have a green tinge. Raised bumps can form on the fins. The fungus should be removed by rubbing the area with a swab and then an antibiotic cream should be applied.


This is a virus which is not highly contagious. It causes discolouration of the skin and sometimes lesions. This condition often occurs following a change of water temperature. The afflicted fish should be quarantined in a tank in which the water temperature is raised. However, sometimes this virus clears up on its own. It can also be treated with products containing neutral Acriflavine.


Epistylis is a rare parasitic infection that can cause other diseases in your Koi. It is usually the result of poor water quality. The parasites look like a fungus and promote the formation of white tufts around wounds and ulcers. You should quarantine the affected fish, change the water in the pond and increase the concentration of salt in the water.

Skinny Disease

This disease is caused by a bacterial infection. The affected Koi exhibit a sucked-in gill appearance and an enlarged head. This condition can be addressed by providing additional food or by adding erythromycin to the feed.

Carp Pox

Carp Pox is a common condition and not particularly contagious. It is rarely fatal but may disfigure the fish. It causes waxy raised growths which are pinkish in colour. There is no treatment for Carp Pox and it usually clears up on its own but heating the water may help to speed recovery.


This disease is also known as Hole in the Head disease and is caused by a protozoan parasite. Koi may become lethargic and skin lesions can develop. An affected fish will often seek isolation in the pond. The afflicted fish should be quarantined and treated with Metronidazole or medicated food.


Flukes are incredibly small and so you will need a microscope to verify their presence. Dactylogyrus or gill flukes attach to the gills whilst gyrodactylus flukes attach to the body. They are parasites which feed on, and therefore erode, the slimy coating on the skin of the Koi. This exposes the fish to infections and causes irritation. If you have specimens afflicted by flukes then the entire pond could be affected and must be treated with a proprietary product containing Praziquantel. You should routinely treat your water in the spring and autumn.

Velvet Disease

This is a condition caused by Oodinium parasites. Your koi will acquire a velvety golden dusting and may lose scales. This is a rare condition which can be treated by adding Formalin to the water.


Like flukes, Costia are tiny parasites which reproduce rapidly. Koi generally do not suffer from Costia unless they are already debilitated. If fish are affected then this usually happens in spring. Koi will seem lethargic and will try to rub against the sides of the pond. The affected areas take on a white/grey hue and the Koi's fins may redden. If the parasites afflict the gills the fish may be seen gasping at the surface of the water. Costia can be treated by adding Malachite Green and Formalin to the pond but you must ensure that there is no salt in the water prior to treatment. You could also consider adding Acriflavine. The affected fish can be given a salt bath to aid recovery.

Pop Eye

This condition is also known as exophthalmia. It is a response to excessive fluid or gasses behind the eyes and causes the eyes to bulge. The affected fish should be given a salt bath.


Leeches feed on the koi's blood and will eventually kill the fish if left untreated. They can also transmit the disease SVC. The leeches will be visible on the body. Proprietary treatments in powder form are available.

Saprolegnia Fungus

This is a common fungal infection found in Koi. Spores from this fungus can grow on any part of the fish. The fungus attacks the Koi by germinating on any dead tissue. The juices released to break down this tissue will eventually start to kill living tissue. The fungus looks a little bit like cotton wool and usually only attacks fish with damaged skin. The condition can be addressed by quarantining the fish in water which is at least 77°F and by raising the salt level to 0.3%.


This is a protozoan parasite which causes Koi to display a grey/white opaque appearance. The parasite damages the Koi's tissues and can afflict the skin and the gills. The fish may appear lethargic and will rub themselves on the side of the pond. Trichodina can be treated by raising salinity to 0.6%. A course of Formalin may also be required.

Koi Herpes Virus (KHV)

This is a potent virus which results in sloughing off of the skin. This sloughing can then leave the fish open to infections. Your fish will be lethargic and will develop lesions on its skin, gills and fins. KHV must be treated in haste by increasing salinity to 0.45% and raising the water temperature to 87°F. Medicated food may also be required.

Gill Maggot

This parasite often attacks the Koi's gills and has a maggot-like appearance. It causes irritation to the fish. Seek out products effective against maggots and increase aeration to address the issue.

Bent Koi

You may notice that your koi are taking on a rather bent appearance. This could be the result of scoliosis which is caused by a deficiency of ascorbic acid in the diet. Scoliosis can be treated by feeding foods high in Vitamin C. The condition can also be due to electrical discharge in the water. This would be caused by damaged submerged pumps. Bent Koi might also be the result of an air bladder infection. This will probably require antibacterial injections.

As you can see they are many conditions which could afflict your fish. It can be difficult to identify specific conditions as there are symptoms common to several diseases. If in doubt, seek expert advice. We have only provided a brief explanation of each condition and this may be insufficient to enable you to make an accurate diagnosis.

At Krafty Koi we have a great range of to help tackle diseases that may come to affect your stock.

Back to blog

Leave a comment

Please note, comments need to be approved before they are published.