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Koi Parasites Costia

Costia or Ichthyobodo necatrix, is a ciliated protozoan parasite of freshwater fish that also has the capability to kill koi in great numbers, and in no short time-span. The only good fortune in this is that most Costia perishes readily when salted. Costia may be attached, or free swimming. Attached Costia look like little commas stuck into the skin (or gill) by the thin end. Free swimming Costia are graceless wobbly swimmers that look like commas or almost like half open Conch shells.

In most cases, Costia clears easily with salt, and this infection should be suspected when a lot of fish are dying, fins may be reddened, and it appears that the fish cannot breathe very well. Spider web lesions in rapidly dying fish are also characteristic as well as excess mucus.

Update: because of the over use or salt, there are now many “salt resistant” parasites. If the parasites do not respond to salt, the recommended treatment would be TERMINATE which is made to treat “salt resistant” parasites.

Costia
Costia is very small under the microscope