Costia or Ichthyobodo necatrix, is a ciliated protozoan parasite of freshwater fish that also has the capability to kill fish 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.
When salt is ineffective in treating Costia, Formalin is recommended.
Costia is very small under the microscope
This video has considerable footage of this parasite under the microscope. The video is immediately available.