Trichodina causes flashing in koi. It is not usually a fish killer on its own. Unlike many parasites, it doesn’t feed on fish; these parasites only use the fish host as a home and means of transport. However, in large numbers, they are incredibly irritating, and some tissue damage will likely result from the sucking disc that trichodinids use for attachment.
It will often cause sores on the fish due to the fish flashing and rubbing its side raw, thereby allowing bacteria to infect the abraded area. Trichodina can cause gill damage in smaller koi. Trichodina looks like a dinner plate under the microscope, and as it moves, it looks like a flying saucer.