Chilodonella – “Killer Don” as referred originally by Dr. Jack Gratzek – this is the one that kills so many fish so fast in the Springtime. Treats easily with, salt. I have never seen an exception. Clears almost overnight, so salting save the lot of fish. Salt levels of 0.3% tend to be quickly curative. I’d recommend looking at the salt article in order to “do it right.”
Chilodonella is one of the hottest fish killers there is.
Under the scope you may see a heart-shaped organism, or a large round organism full of tiny bubbles. Alive, the Chilodonella resembles a heart shaped onion, with a fuzzy end where you could imagine the onions roots would be. These are actually cilia. Dead, Chilodonella are motionless round balls full of tiny bubbles. They may resemble Ich but they do not have a crescent nucleus nor do they move in their dead, rounded state.
Chilodonella clears EASILY with salt. Leave the salt in for 14 days, and be sure to supplement aeration, as gill damage d/t Chilodonella may be severe in the survivors. Chilodonella should be suspected anytime large numbers of fish are dying on the surface, or who roll over on their sides except when disturbed, the Koi dash madly.
Here’s a video on Chilodonella