Chilodonella is one of the fastest fish killers there is. It can cause large numbers of fish to die on the surface of the pond. Often times fish infected with Chilodonella will become lethargic and even begin to roll over on their side only to quickly upright themselves and swim off when disturbed. Under the microscope they resemble the shape of a mans ear and move around acting like a bumper car as they run into things.
One of the reasons it can be such an effective killer is that unlike many parasites, it tolerates a wide variety of different temperatures. That means that Chilodonella can still be a threat to koi even at very low temperatures when your fish will be least likely to fight it off.
It can be identified through skin scrapes and gill biopsies from its characteristic slow gliding movements, often turning in circles. Chilodonella can be treated a couple of different ways. Malachite Green and Formalin is the most effective, but potassium permanganate can also be effective. Potassium Permanganate is unfortunately much harder on your fish than treating with Malachite Green and Formalin.
Malachite Green and Formalin should not be used when the salt level in the pond is higher than .1% unless you know for a fact that the pond water is very soft. If the pond water is hard at all and the salt level is above .1%, the use of formalin can severely burn the gills of the koi. Malachite Green & Formalin can kill the beneficial bacteria in the biological filter, so if possible, the filter should be bypassed for at least 8 hours during use. Malachite Green & Formalin can kill plants so, if possible, all plants should be removed while the pond is being treated and the plants should be treated with potassium permanganate separately before returned to the pond. (To treat pond plants for parasites, mix up a solution of 3 tablespoons of potassium permanganate dissolved in 12 gallons of water. Allow the plants root ball to dry out slightly and then completely submerge the plant root system in the solution for 2 hours.)