Parasites are organisms that live temporarily or permanently on or in another living being (host), feed on it or attack it for reproductive purposes. The parasites can be roughly divided into two groups: tiny single-celled organisms and larger multicellular organisms.

Basically, there are parasites in every pond. For the healthy Koi, however, they do not pose a problem, as they can easily cope with a normal density of parasites as long as the ambient conditions are right. If the fish are excessively attacked by parasites, the following reasons are usually responsible:

– poor health,
– poor nutrition,
– too much stress, e.g. B. by too high a population density,
– poor water quality or
– too cold water.

The koi is most often attacked by ectoparasites. These colonize the skin of the fish and are perceived by them as very annoying. They feed on skin particles and irritate fish skin. The injuries lead to discolouration and increased mucus formation. Once the mucous membrane is damaged, fungi and bacteria have an easy time. Sometimes whole fish stocks die as a result. The best-known parasitic unicellular organisms in the koi pond are Ichthyo, Chilodonella, Trichodina and Costa.

The smaller multicellular cells that can only be seen under the microscope include the gill worm (Dactylogyrus) and the skin worm (Gyrodactylus). Multicellular ectoparasites such as stick crabs (Lernea), gill crabs (Ergasilus), carp louse (Argulus) or the leech can be recognized with the naked eye. Inner parasites such as Spironucleus or Hexamita, which can be detected in the faeces using a microscope, or tapeworms, whose eggs are also found in the faeces, are rarer in Koi.
Parasites such as Trypanosoma can also settle in the blood of the Koi. These parasites are transmitted by leeches, in which they also multiply.

Parasite infestation (in general) with Koi is most likely in cold ponds, which warm up again in spring. At this time of year, the fish are still weakened from winter and their immune defence is not yet fully mobilized.