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