Costia is a ciliated protozoan that can kill high numbers of fish in a short time, just like Chilodonilla. It can be active in cold water down to 45 degrees Fahrenheit. Small quantities of Costia are not uncommon and appear not to be detrimental to the fish’s health. In small numbers, these parasites seem to live on cellular debris in a coexistent relationship with their fish host.

It becomes a severe threat when, for various reasons, the parasite becomes established in large quantities. As with all parasite infestations, large numbers will affect fish health by causing severe tissue damage to both skin and gills, and secondary effects such as hyperplasia or secondary infections – particularly of the gill. What makes matters worse is how this parasite reproduces. Since costia 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 biopsy. It is shaped like a comma and wobbles as it swims. Under the microscope, it can be challenging 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 millimeter). Because they are small, it sometimes helps to rack down the microscope condenser and add a little more contrast.

Treatment Procedure with Malachite Green and Formalin

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 beneficial bacteria in the biological filter. If possible, the filter should be bypassed for at least 8 hours. Malachite Green & Formalin can kill plants, so, if possible, all plants should be removed. At the same time that the pond is being treated, the plants should be treated with potassium permanganate separately before returned to the pond.

Note: To treat pond plants for parasites, mix up a solution of 3 tablespoons of potassium permanganate dissolved in 12 gallons of water. Allow the plant’s root ball to dry out slightly and then completely submerge the plant root system in the solution for 2 hours.)

  • To treat ponds up to 1500 gallons. Measure out enough Malachite Green & Formalin to treat the whole pond at a dosage of 10 ml per 100 gallons. Drain 50% of the water out of the pond and then add the Malachite Green & Formalin to the pond. After exactly 2 hours, refill the pond as fast as possible to its average level. You must be able to refill the pond within 45 minutes. Otherwise, you risk burning the fish’s gills. Perform this treatment three days in a row. If you have costia, we recommend treating one more time on day five. If the pond is being refilled with city water, remember to add dechlorinator to the water to remove any chlorine in the new water. This technique is very effective because the parasites are exposed to twice the dose of medication for 2 hours, which is usually enough to kill them.
  • To treat ponds over 1500 gallons. The above treatment cannot usually be performed on ponds over 1500 gallons because it takes too long to refill larger ponds. In this case, we recommend performing a 25% water change before each treatment. After the pond is refilled to its average level, add the Malachite Green & Formalin at a dose of 10 ml per 100 gallons. Treat the pond three days in a row. If you have costia, we recommend treating one more time on day five. If the pond is being refilled with city water, remember to add dechlorinator to the water to remove any chlorine in the new water.

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