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Saprolegnia

(Cotton wool disease)

Affected fish

All fish.

Cause

The fungus is present in aquariums and feeds upon dead organic matter, and is a normal part of the background fauna. Healthy fish produce a mucous layer, which will usually prevent the fungus infecting the fish. When the mucous layer is disrupted (e.g. by wounds or damage from other disease) fish can become infected with the fungus.

Symptoms

A cotton wool like growth of fine, white strands is the most obvious symptom, normally occurring on the body around wounds. The fungus is sometimes confused with excess mucus production caused by another irritant, which also appears as white strands flowing off the fish. Dead fish often become covered by the fungus, as does leftover food, although this may occur without living fish being infected.

Prevention

Keep waste organic matter low by removing dead material and carrying out regular maintenance. Keep a close watch on any wounds for infections.

Treatment

Treatment is relatively easy and can be done with proprietary fungal treatments. Malachite green is a commonly available treatment that should be effective. Adding aquarium salt to the water at a low level of 1-3 grams per litre can be very effective.

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