Cotton wool disease and mouth fungus on fish

Cotton wool disease and mouth fungus on fish

Symptoms of cotton wool disease/mouth fungus in fish

Mouth fungus is not a fungus but cotton wool disease
Mouth fungus in fish is not a fungus but cotton wool disease

It might look like fungus but this disease is actually caused by bacteria. The bacteria is called Flavobacteria.
The first signs of this disease are greyish white marks around the mouth, or on the body or even on the fins of the fish. Fungus on the other hand will start off fluffy and light.
When the infection spreads the result is a cotton wool like growth around the mouth, body or fins. There will be red ulcers where the infection is penetrating the skin and the fins will be frayed or eaten away.
The fish will stop eating and will swim in a laboured manner, shimmying as it swims. The fish may become reclusive as well.

Make sure you make the correct diagnosis. If you get it wrong you will not be giving your fish the correct treatment.

Causes of cotton wool disease/mouth fungus in fish

Cotton wool disease is caused by bad or wrong water conditions. For example, not removing chlorine from tap water or keeping soft water fish in hardwater or a build up of fish waste causing ammonia in the water which harms the fish. Good biological filtration and weekly water changes are preventative measures.
Biological filtration – cycling

Treatment of cotton wool disease/mouth fungus

Cotton wool disease is infectious so it is important to remove all affected fish to a quarantine tank and treat there. Dose the old aquarium and the quarantine tank with some methylene blue to kill off any remaining bacteria.

Treatment is best carried out early before the infection spreads internally. Immediately treat the affected area with a cotton wool bud dipped in a bactericide and more importantly fix the poor water conditions in the aquarium that led to the outbreak of cotton wool disease.

Try siphon through the gravel to remove excess fish waste and rotten food. Squeeze out excess mulm from the filter’s sponge to ensure a good flow of water through the filter. An immediate change of 25% of the water from the aquarium should help. Make sure the new water is dechlorinated and of the same temperature.



1 thought on “Cotton wool disease and mouth fungus on fish”

  1. Hello I set up my tank in October 2016. 2 months later after fish less cycle Added 6 neons when all the tests were right.One of my tetras has had a white mouth for about 3 weeks. Have used esha 2000 for 3 days. No change after 2 weeks so carried on dosing again with esha 2000. Little tetras mouth is the same, have noticed after eating his belly looks swollen but after a while it looks normal. The rest of my fish are fine
    I have set up a hospital tank but don’t want to put him in there as its not cycled yet. Have also bought methalyne blue.
    Should I add the methalyne blue to the tank that contains all the tetras, I also have a little algae eater!
    Tank size 61x31x31 = 58 litres
    Temperature. 24.5 Celsius
    Water test yesterday:
    Ammonia 0
    Nitrate 0
    Nitrite 0
    Thank you


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