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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.
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.