There are many parasites which can infect a fish's gills, causing various diseases and disorders in these organs. The fish usually becomes pale and has difficulty breathing. Two common parasites which infect the gills of the fish include Dactylogyrus and Neobenedenia.
Dactylogyrus is a gill parasite which most commonly infects koi, discus and goldfish, and appears like a small worm under the microscope. Infected fish display such symptoms as swollen and pale gills that make breathing difficult for the infected fish. A fish with dactylogyrus will try to remove the parasite by brushing and rubbing against objects in its environment.
Neobenedenia is a large parasite which infects saltwater fishes and destroys their gills, leading to breathing problems. Unfortunately, a Neobenedenia infection often becomes fatal for the fish.