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 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. Chilodonella is identified through skin scrapes and gill biopsies. Under a microscope, it resembles the shape of a man’s ear and moves around, acting like a bumper car as it runs into things. It has characteristic slow gliding movements, often turning in circles.
One of the reasons it can be such an effective killer is that, unlike many parasites, they tolerate 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 are least likely to fight them off.
Chilodonella is treatable in a couple of different ways. Malachite Green and Formalin are the most effective, but potassium permanganate can also be useful. Potassium Permanganate is, unfortunately, much harder on your fish than treating with Malachite Green and Formalin.