Costia
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.
A high magnification must be used to view these parasites (300 x) and staining is recommended for positive identification.
Recommended treatments include Potassium Permanganate, Acriflavine and strong salt baths of 3% ( 4 and one half oz. per gallon )