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There are many skin parasites that infect fishes. One such parasite is the anchor worm. However, despite its name, it is not a worm, but a large, parasitic crustacean of the Lernaea species. And although the parasite can infect any fish, pond fishes are most susceptible. Once attached to the fish's skin, it buries its head deep into the muscle tissue.
A fish infected with anchor worms will have red and inflamed skin irritations. Take a closer look, and you can see the parasite's body sticking out, appearing like whitish-green threads. The fish will also rub or brush itself against objects, in an attempt to get rid of the anchor worm(s).