Koi Parasites Trichodina
This saucer-shaped parasite is over-rated in its pathogenicity, although I have seen it cause spider web lesions in the skins of Koi. The most serious damage it may do is to the gills of smaller fishes. This is one of two or three parasites that causes scratching in Koi, with no serious side effects. The most important thing about Trichodina, is that when it is discovered, it belies a problem in the System 
Housekeeping department. You can bet your license that there is some accumulation somewhere of mulm, and lots of it. TRICHODINA clears easily with SALT, and in very short order. You need only leave salt in for a few days to clear Tricho for good. Here again, if salt cannot or will not be used, TERMINATE is the next choice.
Update: because of the over use or salt, there are now many “salt resistant” parasites. If the parasites do not respond to salt, the recommended treatment would be TERMINATE which is made to treat “salt resistant” parasites.
Trichodina looks a little like a Frisbee or flying saucer under the microscope.