Ichthyobodo necatrix (Costia) - Koi Parasite

This is a microscopic protozoan parasite. Ichthyobodo necatrix also known as Costia.
Costia causes great harm and will even kill a Koi fish if left unchecked. Costia is the smallest parasites that attack Koi and the hardest parasite to see. Life cycle of Costia is very short and still be active in cold water. On the microscope slide look for a group of swarming commas. Costia can cause a secondary infection, like Aeromonas.