Costia is a minute Flagellate with 3-4 flagella. It affects both the skin and gills of Koi, and reproduces itself by binary fission. Infestations of this parasite can appear very rapidly indeed, and Koi suffering infestations exhibit the classic symptoms of lethargy, clamped fins, rubbing and flashing and the skin can take on a grey white opaqueness.
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.
Costia is one of the many fish diseases that are caused by protozoa. It is a somewhat rare disease that is relatively easy to cure if your fish becomes infected by it. There are several treatments for this disease but it is of course preferable to prevent the protozoa from ever infecting you fish by keeping you aquarium clean and your fish healthy and well fed, while at the same time doing what is possible to avoid introducing diseases and parasites into the pond.
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 )