Fish, Tanks and Ponds

Fish, Tanks and Ponds
A comprehensive guide to fish


Hatchetfish with fungus
Hatchetfish with fungus

Pre-disposing factors

As with most health problems stress due to poor conditions is a major underlying cause for fungus. A small trauma to a fully healthy fish is likely to heal without any intervention but the same trauma on a fish under a chronic stress could is far more likely to result in a fungal infection. Large traumas on any fish could result in a fungal infection but what ever the initial cause it must be identified and removed or the treatment may fail.

Treatment There are several stages to successfully treating a fungal infection.

  1. If possible (due to the size of the fish) physically remove as much of the cotton wool like fungus as possible using a soft swab which has been dipped in a solution of Povidone iodine. This should be carried out daily if necessary.
  2. Prepare a bath containing 2mg/litre of malachite green and place the fish in it for 45 minutes. This should be repeated every other day for three treatments. Alternatively prepare a long term bath containing a 0.1 to 0.3% solution of salt. Using salt as a treatment has the advantage that it will reduce osmotic stress which will occur when ever a fish has a large wound.
  3. Once the fungus has been treated and there are no signs of it left, carry on treating the lesion left behind from the fungal attack with a topical solution of povidone iodine until healing begins. This will prevent the wound from becoming re-infected as well as preventing any other opportunistic secondary infections.


If the underlying cause is dealt with and treatment given in time (as early as possible) then the fish should make a full recovery.

As with most other fish ailments early intervention is essential. Very well established fungal infections are very difficult to treat and they will cause serious secondary problems making successful treatment less likely.

See Treatment finder

Cotton wool disease

Saprolegnia is easily recognised by its cotton wool like appearance. Although it may appear to be a relatively harmless condition Saprolegnia can easily and quickly prove fatal if untreated. Unlike plants and animals fungi are not able to provide their own food, they feed on dead or living organic matter and they can't be eradicated completely because they are always present in the background.