Fungus, often called 'water mold' is a common parasitic disease affecting all species of fish. The most common fungi species affecting fish are of the genera Saprolegnia, Achlya, and Dictyuchus. Fungi infestations are generally considered to be secondary infections that occur following an injury or in association with poor environmental conditions.

Signs of fungal infections include cottony or fuzzy appearing growths usually associated with discolored areas or lesions. The fungal growths contain flagellated spores that are capable of moving out and infecting other fish. If left untreated, fungal infestations will spread to all fish in an infested rearing unit.

Eggs should be treated daily with a flow through treatment of formalin at 1,600 2,000 parts per million for 15 minutes to prevent fungal infestation. Hydrogen peroxide can also be used. Fisheries Division is also currently evaluating non-chemical approaches to treating eggs. An effective treatment for fish is a flow through treatment of a 0.5 1.0 % salt (sodium chloride) solution for 15 minutes repeated daily as needed.