There are many types of parasites that can infect a fish's skin. Two such microscopic parasites are Dactylogyrus and Gyrodactylus. These are tiny flatworms, which infect goldfish, koi and other types of fish.
Symptoms and Types
Fishes infected with these parasites will display sores and ulcers on the skin and gills, have a pale skin color, and be covered with small hemorrhages. Dactylogyrus and Gyrodactylus infections should be diagnosed and treated quickly because they can become fatal for your fish.
Veterinarians will prescribe medicated water with formalin and praziquantel. Once the fish is placed in the water, the medications should eradicate any parasites.
The fish's environment should be cleaned and sanitized to prevent Dactylogyrus and Gyrodactylus infections. And any new fish should be quarantined or isolated before it is introduced into an environment filled with healthy fish, especially if the fish is suspected of having such infections.