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Koi Diseases (What To Watch Out For)

Koi Diseases

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Koi are hardy fish that can withstand various water temperatures. Still, poor water conditions may cause stress that weakens your koi’s immune system, eventually setting the stage for various diseases. There are several koi fish diseases most commonly known to breeders, but there are also many other lesser-known illnesses that can also be problematic. This may have you wondering how to tell if your koi fish is sick. So, what should you know about koi diseases?

Clinical signs of koi diseases include lethargic behavior and skin lesions. Most koi diseases are parasitic in nature. These parasites thrive in poorly maintained ponds and eventually feed on your koi fish’s tissues. This article highlights the most common koi diseases and treatment methods for some curable diseases.

Koi Diseases and Symptoms

Koi Diseases and Symptoms

Koi diseases come from either parasites, bacteria, or worms. When any of these causes infects your koi fish, they will appear lazy and may develop white spots on their skin. Healthy koi fish are active and have vibrant colors.

You want your koi to remain vibrant and active in your pond, so be sure to maintain an optimal pond condition. Any water temperature or water pH level that is less than ideal will become the perfect breeding ground for koi diseases. Here are common koi fish diseases you risk with an unkempt pond.

Parasitic Diseases

You can usually tell your koi fish have parasites when they develop white patches or spots on their skin. Sick koi fish also swim near the surface to gasp for air because parasites typically infect gill areas. Here are common parasites that may infect your koi fish:


White spot disease or ich is a common parasitic disease among tropical and pond fish. Ich starts as cyst hatchlings at the bottom of unmaintained ponds and then swims to find a fish host.

This parasite feeds on koi tissues and may appear like small grains of salt on your koi’s skin. You can treat ich by isolating your sick koi in a holding tank with increased salinity levels (0.5%-0.6%).


This protozoan parasite has thread-like structures it uses to dig through your koi’s mucous layer and feed on koi tissues. Koi fish’s mucous layers may become susceptible to trichodina attacks when your pond water quality is poor.

Infected koi will have white or gray patches on their skin and may exhibit flashing behavior. Flashing is when your koi suddenly swims in bursts in an attempt to scratch themselves.

You can treat this disease the same way you would treat ich with increased salinity. However, parasites may develop immunity to increased salt levels, so you should include formalin or malachite green in your quarantine tank water solution.

Fish Lice

Argulus or fish lice are large parasites that may attach themselves to your koi fish’s mouth, gills, or skin. Infected koi may develop a bacterial infection from damage due to the parasite’s hooked appendages.

Fish lice cause severe irritation in koi that may cause your fish to exhibit flashing and rubbing behavior. Constant rubbing against pond walls for relief may further damage your koi’s skin and develop more infections.

Fish lice are large enough to remove physically, but the organophosphates are the most effective treatment. Common organophosphate brands include Dimilin and Lufenuron.

Bacterial Diseases

Frayed tails or fins, visible swelling, lifting scales, or white threads across your koi’s mouth are some signs of bacterial infection. Bacterial damage is more so secondary in effect due to poor pond maintenance. Here are some examples of bacterial koi diseases:

Fin Rot

Any form of rot in your fish is due to the pre-existing bacteria in your pond. Your koi fish may be immunocompromised due to poor water maintenance, thus weakening your koi’s immune system. The bacteria damage your koi’s fins, tail, and mouth.

Proper pond maintenance is the best fin rot treatment. Change up to 50% of your pond water and add products like Aquarisol or MELAFIX, which are specifically designed to deal with natural pond bacteria.


Pinecone disease or dropsy infects koi fish when there is overcrowding or poor pond water quality. Infected fish appear bloated, with scales lifting. Bulging eyes are other signs of dropsy. Untreated dropsy may result in kidney and liver failure in koi.

If you suspect your fish suffers from dropsy, immediately quarantine your koi in a saltwater solution for at least ten days.


Cottonmouth disease in koi is due to columnaris infection. This bacterial infection looks like white threads across your koi fish’s mouth, hence its cottonmouth nickname. Infected koi fish will appear slimy with soggy bellies.

Just like other bacterial infections, you should quarantine your infected koi as soon as you notice any sign of infection. Consider treating your isolation tank water with potassium permanganate to help treat cottonmouth disease.

Worm Diseases

Worms infect koi fish in your collection when you add new fish without conducting proper quarantine procedures. New koi fish may be carrying premature or juvenile worms from your primary koi provider. Here are some examples of worms that may infect your koi fish:


These microscopic flatworms attach themselves to your koi fish’s gills or skin. They resemble parasites that cause itching in your koi. Infected fish will attempt to scratch themselves by flashing or scratching against pond walls for itch relief.

Potassium permanganate is a standard medication to treat flukes. Consider treating your entire pond if you notice flukes in one fish because it’s highly likely that your whole pond has fluke eggs. This possibility highlights the importance of proper quarantine methods before introducing new fish to your main pond.

Anchor Worm

Lernea or anchor worms are crustacean parasites that dig into your koi’s skin right through their mucus layers. Like parasites, these worms feed on koi tissue.

These worms are large enough to remove with tweezers. Once you are positive that you have removed anchor worms from your koi, treat your pond with Dimilin like you would for fish lice.

Conclusion: Koi Diseases: What You Should Know

What you should know about koi diseases is that they are mostly secondary effects due to poor pond maintenance. You must keep your pond at optimal levels at all times. Be sure your pond offers your koi the best environment.

A stressful environment for koi would have poor water chemistry and overcrowding. Stressed koi fish become immunocompromised, thus becoming vulnerable to many koi fish diseases. Be sure to observe the best practices in raising koi to avoid any diseases.

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