Koi Parasites

Anchorworm (Lernea)

Anchorworms resemble a short piece of thread coming from beneath a single scale. Koi infected with Anchorworms very often develop ulcer sores due to the damage caused by the worms as they bury into the flesh of the fish. Be aware that anchor worms will not disappear immediately. The treatment may take a week or longer, according to the water temperature.

Carp Pox Virus

Is an unsightly but relatively harmless virus seen occasionally on koi. It has a waxy white appearance. It's mostly seen in colder water temperatures and will many times disappear as the water warms.

Columnaris (Flexibacter Columnaris) Disease

Columnaris (Flexibacter columnaris) Disease is another bacterial infection. The primary symptom is white tufts that develop around the mouth and spread to the body and fins, often leading to ulcers and a thin appearance.

Often mistaken for a fungal infection because of its mold-like lesions, Columnaris is a common bacterial infection in cultured fish, particularly livebearing fish and catfish. Its name is derived from columnar shaped bacteria, which are present in virtually all pond environments.

The bacteria are most likely to infect fish that have been stressed by such conditions as poor water quality, inadequate diet, or handling and shipping. Columnaris can enter the fish through the gills, mouth, or via small wounds on the skin. The disease is highly contagious and may be spread through contaminated nets, specimen containers, and even food.

Treatment with anti bacterial medicine is usually effective.


Costia are microscopic parasites that attack the skin and/or gills of the fish. Symptoms include loss of appetite, listlessness, flashing (rubbing their sides against the pond bottom), and excess slime production.

Costia normally only affects fish that have already been debilitated by some other cause, and can often be seen on Koi as a secondary parasite.

Costia is one of the many fish diseases that are caused by protozoa. It is a somewhat rare disease that is relatively easy to cure if your fish becomes infected by it. There are several treatments for this disease but it is of course preferable to prevent the protozoa from ever infecting you fish by keeping your facilities clean and your fish healthy and well fed, while at the same time doing what is possible to avoid introducing diseases and parasites into the pond.


Raised scales (rather like a pine cone) and eyes standing out from the head. A sign of a number of conditions, may be congenital heart or kidney failure or an internal bacterial infection.

Bacterial dropsy is infectious so treat with an anti bacterial remedy and if possible isolate affected Koi.

Fish Lice (Argulus)

Fish lice are sometimes mistaken for a small patch of green algae on the fish. After looking closer, you will see that it is a round "bug" that is a little larger than a pencil lead. As with anchor worms, fish lice make little holes in the fish, making them vulnerable to bacterial infections.

Flukes (Gyrodactylus and Dactylogyrus)

Flukes are the most common parasite found on koi and goldfish. High numbers can cause serious damage. Symptoms include flashing (rubbing their sides against the pond bottom), gasping at the surface, and frayed fins. Most cases of ulcer disease are preceded by fluke infestations.

Ich (Whitespot)

Ich is one of the most common and widespread of all fish diseases. It is characterized by tiny white spots the size of coarse sand. Its microscopic stage is round with a horseshoe shaped nucleus. Symptoms may appear before you see the white spots. They include flashing (rubbing their sides against the pond bottom), lethargy, and loss of appetite.

The treatment of choice for ich is salt for 10-21 days, depending on the water temperature. Ich takes longer to clear in colder water. 

Ulcer Disease

Ulcer disease is a particularly destructive bacterial disease. Seen on koi and goldfish, it starts out as a red or white pimple and quickly widens into a large hole or ulcer. It can advance all the way into the muscle of the fish. It is very important to catch this disease in its earliest stages.


Another microscopic parasite, trichodina is easily identifiable. It resembles a wagon wheel. Fish infected with trichodina often develop a gray film over the body in response to the infestation. Other symptoms include flashing (rubbing their sides against the pond bottom), lethargy, and hanging at the surface.