Occurrence of fish lice in goldfish is very rare. Often disk-shaped parasites can be found attached to the skin of the goldfish. Ulcers can be sighted near the point of parasitic attachment. Bacterial or fungus problems may follow.
Fish lice is caused by a crustacean parasite. After feeding on the skin, the adult parasite will leave its host and lay gelatin-like caapsules full of eggs. The eggs will not hatch until the temperature rises, and may stay in the tank for long periods of time.
Treatment involves the removal of the parasite from the fish with a small pair of tweezers. The wound is then be treated with a cotton swab that has been dipped in Mercurochrome. The tank and all equipment including decorations must be completely steralized to completely destroy this parasite.