Ichthyobodo: AKA Costia
Costia is a microscopic parasite. It is not uncommon to find very small populations of Costia on some koi or pond fish.
However, the problem begins when these populations grow into large numbers which do damage to the gills and skin of your koi or pond fish.
Plus, they also cause secondary bacterial infections. Costia reproduce by a process called binary fission “they divide in two”.
Give them the right high temperature and they can double every few hours. Costia can wipe out a collection of koi and pond fish very quickly.
Sick fish, poor water conditions, overcrowding and poor management. Most parasites will die if you dry out your nets, hoses and other pond tools, NOT Costia! Costia will infect one group of fish to another even on dried pond tools and tanks.