Payless Pet Products
Your Subtitle text

Fungal Infections

Fungal infections are among the most common diseases seen in tropical fish. Because fungal spores are found in all fish tanks, they can quickly colonize and create problems in stressed, injured, or diseased ornamental fish. Poor water quality can also lead to an increase in fungal infections in an otherwise healthy fish population. Most aquarium owners can easily identify external fungal infections. Most fungal infections have a characteristically fluffy white appearance and are this fungal infection is commonly known as cotton wool disease. As the fungal infection worsens, they may take on a gray or even red colored appearance.

Fortunately most fungal infections only attack the external tissues of fish. Most infections are usually associated with a pre existing infection or injury and this is why treatment is often necessary to completely cure these infected fish. There are a few fungal infections that will infect the internal organs of fish.

Fungi are present throughout most aquariums, but certain conditions increase the possibility of fungal infections including

Poor water quality
Poor hygiene
Dead fish or large amounts of decomposing organic material in the tank
Fish that are injured, old, or have other diseases

Tanks with fish that have re occurring fungal infections need to be examined for tank hygiene, filtration and the quality of the water. Healthy tanks should rarely have fish with fungal infections.

Common Fish Fungal Infections are:

Cotton Wool Disease

Cotton wool disease is a general term applied to the most common fungal infections that infect the skin, fins and mouth. The fluffy white growths often colonize areas where there have been previous infections, parasites, or injuries. The most common types of fungus in these infections are Saprolegnia and Achyla. Other fungi may also cause these infections and there may be more than one species at the site of an infection.

Egg fungus

If the aquarium owner has fish that are actively breeding, the eggs can become infected with a fluffy white fungal infection. The Achyla and Saprolegnia fungi are the species that cause the most problems. These fungi are present in most aquariums and will often infect damaged, diseased, or infertile eggs, and the fungus can then spread to healthy eggs. Since many egg laying species protect their eggs and will pick out diseased eggs, this is primarily a problem in situations where the parents have abandoned the nest or in artificial rearing situations. Once an egg has become infected there is no practical treatment. Infected eggs should be immediately removed with a pipette, needle, or forceps. It is important the aquarium owner does not confuse a healthy opaque egg with a fungally infected egg.

Gill rot

Gill rot is an uncommon fungal infection but if it does occur, it can be deadly if not treated. An infected fish usually gasps for air and has gills that are covered with mucus and are mottled in appearance. The infection is usually caused by the fungus Branchiomyces and can cause the entire gill to rot away. Infections usually occur in stressed fish that are living in tanks with high levels of ammonia or nitrate.

Systemic fungal infections

Systemic fungal infections in tropical fish are rare and generally considered to be difficult to diagnose and treat. As a result, not much is known about these conditions. A fungus that can cause systemic infections is Icthyophonus. Infected fish show generalized poor health. Diagnosis is often made only through a thorough post mortem examination. Stressed fish in poor quality water and environments are more susceptible to this disease.


Most aquarium owners will have to deal with a fungal infection at one time or another. Most infections can be successfully treated if caught early. Fungal infections are notorious for developing in unhealthy or injured fish, particularly in poorly cared for tanks. If you see a fungal infection in your fish, make sure to check the water quality and insure that your tank is providing a healthy, safe, natural environment for all of your fish.