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Fish diseases


Anchor worms are one of the most notorious parasites that can attack our fish. They attach themselves to the skin tissue with hooks. As the anchor worm bores into the fish, this parasite also causes a wound that can cause an additional infection of bacteria or fungi.

Blood leeches

The leech which is most commonly found as a parasite on Koi is the fish leech (Piscicola geometra). This leech has a slender shape and can reach a size of about 5 cm. Once fish leeches are attached to the Koi, they penetrate the skin of the fish to suck the blood. If a Koi is covered in leeches, its health may deteriorate rapidly and anaemia may occur. Moreover, the Koi can become more susceptible to many other infections

The fish or carp louse (Argulus)

It is a disk-shaped crustacean of which adult specimens can reach a diameter of 10 mm. It is a very common parasite. Both larvae and adults have a spine with venom glands between the eyes. They pierce the skin of the Koi and suck blood as food. Reddish inflammations appear which, if not taken care of, can lead to secondary infections. Fish lice can cause so much irritation that Koi scrub themselves and even jump out of the water.

Skin flukes (Gyrodactylus)

Gyrodactylus trematodes is a 0.3-0.5mm sucking worm that is viviparous and can reproduce extremely fast. The skin worm puts its hooks into the fish. This causes irritation of the skin. The fish will produce more mucus, which causes skin clouding.

Gill flukes (Dactylogyrus)

The gill worm is oviparous. It damages the gills, which thicken and can no longer function properly.

Round or filamentous worms (Nematodes)

The worms cause severe inflammation of the internal organs both in the larval form and in the adult stage. This disturbs their function. At first, the fish become very emaciated. Afterwards, the fish become lethargic, show signs of exhaustion and lose their appetite.

White Dot

Perhaps the best known disease, is caused by a single celled animal with cilia all around. White Spot gets its chance with reduced resistance, changes in temperature, complete filter change, too fast cold water refill, infection from new fish stock, transferring Nishikigoi in a pond without biological balance. (a filter system that has not been run in). Poor water quality and especially stress are very important factors.


This parasite occurs on the skin and gills. It uses the fish as a means of transport, during which it attaches itself to the fish and damages the mucous membrane. Polluted pond water is also a good habitat for the Trichodina parasite.

Costia or skin clouding

A single cell external parasite. In case of mass infection, we find a few grey spots on the skin, which later become red spots and in a further stage cause bloody wounds. Usually this wound is then covered with a fungus. Sometimes, Costia is also observed on the gills. Fish mortality may be high with this disease, as bacterial infections also result. The fish tumble or scrape themselves on the bottom, and in a further stage they begin to lose weight. The fish become restless and have difficulty breathing, they hang to the surface and gasp for breath.


A single-celled parasite that is oval in shape. In case of a mass infection the parasite is lethal. The parasite is most commonly found on the gills. The infection occurs through immediate transmission. Do you have a weak fish at that time, then the disease can break out.