Costia is a ciliated protozoan that can kill great numbers of fish in a very short time just like Chilodonilla. It can be active in cold water down to 45 degrees Fahrenheit.
Small numbers of Costia are not uncommon and appear not be detrimental to the fish’s health. In small numbers these parasites seem to live on cellular debris in a commensalistic relationship with their fish host.
It becomes a serious threat when, for various reasons, the parasite becomes established in large numbers. As with all parasite infestations, large numbers will affect fish health by causing serious tissue damage to both skin and gills, as well as secondary effects such as hyperplasia or secondary infections – particularly of the gill.
What makes matters worse is how this parasite reproduces. Since costi multiplies by dividing itself in two, its reproductive rate can be staggering and the warmer the water the faster they reproduce.
Costia can be detected by a skin scrape or a gill biospsy. It is shaped like a comma and wobbles as it swims. Under the microscope it can be very hard to find because of its small size. You will probably need a 400x magnification microscope to see these small parasites, as they are only 10 -20 µm long (1µm = 1/1000 millimetre). Because they are so small it sometimes helps to rack down the microscope condenser and add a little more contrast.
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.)