Fungus on an Oscar Fish

Written by emma taylor
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Fungus on an Oscar Fish
The Oscar fish is so smart it can recognise its owner. (Red Tiger Oscar image by Mike Price from

Cotton wool disease is a fungal infection that can afflict your Oscar fish. The most common fungi that cause this disease are called saprolegnia and achlya, though other fungi may also be present in your Oscar's tank. Early detection and treatment are important in bringing your fish back to good health.


Oscar fish that develop a fungal infection have a growth on the skin that looks similar to a tuft of cotton. The colour starts off white but as the infection worsens, it can change to grey, brown or red. Infections occur on the skin, fins or mouth. Your fish may appear to be depressed and lose its appetite.


All fish tanks contain some form of fungal spore. Poor aquarium hygiene, unclean water and low water temperature can all create conditions that cause these spores to multiply and infect Oscar fish. When Oscar fish are mishandled or otherwise injured, the outer layer of the skin can be scraped off, making the fish prone to fungal infections. Fish that are old or have recently suffered a parasite or bacterial infection also are at risk of fungal infection.


Always keep the aquarium clean, removing dead fish and other waste from the tank often. Test water regularly to be sure that appropriate levels of pH, nitrate and ammonia are maintained. Be sure the filtration system is suitable for the aquarium and that the filters are changed regularly. Aquarium owners who practice good tank hygiene and properly maintain their aquariums rarely have to deal with a fungal infection in their Oscar fish.


Fungal infections in Oscar fish are treated by using an over-the-counter fungal remedy containing phenoxyethanol that is available at most pet stores. If all of the fish in the aquarium are infected, the whole tank may be treated. For individual cases, it's best to remove the infected fish to a hospital tank. Gentian violet is a topical antifungal and antibacterial agent that can be topically applied to individual fish.

before treatment, it's advised to do a 50 per cent water change and then avoid water changes during the course of treatment. Once treatment is over, change 50 per cent of the water again.

Egg Infections

The fungus also can affect the Oscar's eggs. An infected egg has the same fluffy white tuft of cotton-like growth that is present on an infected fish. Usually infertile or damaged eggs become infected first and, if left untreated, healthy eggs and adult fish in the tank become infected. Methylene blue is often used by breeders to prevent fungal infection in eggs. Once an egg has been infected, there is no treatment, so the eggs must be removed from the tank.

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