Complete Guide to Koi Disease Identification

Koi are a popular choice for ornamental ponds, known for their outstanding beauty and impressive presence. However, maintaining the health of these fish can be a challenge if kept in below par water conditions. In most cases, various diseases that koi contract either stem from water quality issues or mixing of fish from different sources. (Different sources meaning different dealers, hobbyist ponds, different breeders etc). 

For parasitical problems, we recommend undertaking mucus scrapes that are assessed under a microscope to identify particular parasites, allowing you to treat for exactly what parasites the fish have. This enables far better detection and more effective treatments. 

This guide delves into the common diseases affecting koi, providing detailed information about symptoms, causes, and treatments. We can provide solutions to all fish problems and also offer an on site fish health service where mucus scrapes, water testing and topical treatments can be carried out. 

 1. White Spot Disease

Symptoms: White, salt-like spots on skin, gills, and fins; rubbing against objects; laboured breathing.

Cause: Caused by the parasite Ichthyophthirius multifiliis.

Treatment: Increase water temperature to 25C / 76F to speed up the parasite's life cycle. Treat with WS3 or formalin & malachite and in stubborn cases Methylene Blue can be used

 2. Dropsy

Symptoms: Bulging eyes, swollen body, scales protruding like a pinecone.

Cause: Typically bacterial, indicating kidney failure or internal infection.

Treatment: Often difficult; focus on improving water quality, antibiotic food, Acriflavine and salt baths.

 3. Carp Pox

Symptoms: Waxy, pale growths on skin, typically in colder water.

Cause: A herpesvirus specific to carp.

Treatment: No cure, but lesions often reduce or disappear as water warms.

 4. Aeromonas

Symptoms: Ulcers, red sores, lethargy, fin rot.

Cause: Bacterial infection, often linked to poor water quality.

Treatment: Improved water quality, Acriflavine, Anti Ulcer and Bacteria, Salt at between ¼ - ½ oz salt per gallon (0.16 to 0.32) and Antibiotics. Healing will occur quicker at temperatures above 10 C, so additional heat is also beneficial. 

 5. Costia

Symptoms: Cloudy skin, listlessness, clamped fins, excess mucus.

Cause: A protozoan parasite, often thriving in poor conditions with bad water quality and / or overstocking.

Treatment: Maintain good water quality. Formalin and Malachite @ 15ppm repeated after 7 days. 

 6. Chilodonella

Symptoms: Grey-blue film on body, respiratory distress, lethargy.

Cause: Caused by the Chilodonella parasite.

Treatment: Improve water quality, potassium permanganate or formalin / malachite treatments.

 7. Columnaris

Symptoms: Cottony patches on skin, mouth, and gills; frayed fins; ulcers.

Cause: Flavobacterium columnare bacteria.

Treatment: Antibiotics, improved water quality, and salt baths.

 8. Anchor Worms

Symptoms: Visible worms in skin, inflammation, secondary infections.

Cause: Larvae of the Lernaea copepod.

Treatment: Manual removal of worms, followed by Dimilin or Parazan P.

 9. Argulus

Symptoms: Visible lice on skin, scratching, inflammation.

Cause: Parasitic crustaceans (Argulus).

Treatment: Manual removal, Parazin P treatments.

10. Skin Flukes Gyrodactylus and Dactylogyrus

Symptoms: Excess mucus, flicking / flashing / irritation of the koi

Cause: Poor water quality, Stress, overcrowding

Treatment: Praziquantel or Flubendazole based Fluke specific treatments are most effective. In stubborn cases, treat with Potassium permanganate first to reduce mucus and follow up with a Fluke specific treatment. 

 Diagnosing and Treating Koi Diseases

  • Observation

Regular observation is key. Changes in behaviour or appearance can be early signs of disease.

  •  Water Quality

Many koi diseases are linked to poor water quality. Regular testing and maintenance are crucial.

  •  Expert Consultation

When in doubt, seek advice from a vet or a koi specialist.

  •  Treatment

Follow treatment recommendations carefully. Over- or under-treatment can exacerbate issues.

 Preventive Measures

  •  Regular Maintenance

Making sure that you keep your pond clean and well-filtered, this will prevent any illnesses spreading around your pond and keep your Koi healthy. Water quality is a major part of the koi’s well being and many diseases stem from poor environment

  •  Balanced Diet

Provide a nutritious, varied diet to boost the immune system of your koi. Many Koi foods on the market contain probiotics to enhance the health of your koi. A good quality koi food also prevents minimal waste and in turn helps maintain better water quality. 

  •  Stress Reduction

Avoid overstocking and handle fish carefully to minimise stress.

  •  Regular Health Checks

Regularly check your fish for signs of disease, loss of appetite or changes in behaviour, even if they appear healthy. Testing water regularly for Ammonia, Nitrite and pH is the minimum you should be doing to ensure that the environment is stable. 


Identifying and treating diseases in koi can be challenging, but with careful management and a proactive approach to health and water quality, many common diseases can be prevented or treated effectively. Remember, healthy koi require a commitment to their environment and well-being. Stay vigilant and enjoy the beauty of healthy koi in your pond. Please contact us if you require any help at or visit our store, where we stock all of the koi treatments needed to cure your fish.