Goldfish fungus is caused by the saprolegnia and achlya organisms, usually as a secondary infection occurring after goldfish illnesses such as ulcers or parasites. Fungus can also infect wounds that are a result of injury. In tanks and ponds with good water quality fungus is rarely a problem.
The Symptoms of Fish Fungi
Fungus appears as a cotton wool-like or fluffy growth on the skin and/or gills, and is gray-white in color (see photo). Sometimes excess mucus is visible around an open sore or the goldfish's gills.
How Do I Get Rid of Fungus in the Fish Tank?
When discovered, it is important to make sure the water quality is high. A remedy for fish fungus is available from pet shops and vets, and quick action usually cures the infection. If treatment is unsuccessful there is a chance it is not fungus, but one of the bacterial goldfish illnesses called cotton wool disease. It has a similar appearance, but obviously will not respond to anti-fungal medication.