Fish fungus

Why are your fish suddenly covered in fluffy cotton wool like stuff and what has caused this to happen? Fish fungus! Many aquarists have seen cotton wool-like formations suddenly appear on the body, mouth or fins of their fish ranging from a white to a more yellowish color, with varying degrees of severity and localization. These are fungi of the genera Saprolegnia, Achlya and others. However, mycosis is generally a secondary infection. Pathogenic fungi rarely develop on healthy skin, instead you will find them on dead or damaged tissue or areas of the body subject to necrosis.


Most often, necrosis develops in areas of skin injuries. Fish in the aquarium can get injuries in lots of different places and on all body parts with varying degrees of severity, here are some examples:

  1. Mating behavior of males as well as aggression from one species towards another can cause damage. Both forms of aggressive behavior can damage scales and the skin, as well as leaving injuries on the jaws and the fins of the fish.
  2. Capturing methods and transport can cause trauma.
  3. Some species of fish, such as catfish, occasionally hide behind heaters and, if the heater is installed without a protective cover, the fish can get seriously burned. In these burned areas, tissue will die and mold-like growths can develop.
  4. Bacterial and protozoal infections that lead to skin lesions can also result in mycosis.
  5. In places where parasites attach (such as Lernaea), inflammations and fungal growth are often observed.
  6. Unfertilized fish eggs die and are regularly covered in fungus which then in turn can also infect the eggs that are alive by proximity.
  7. Outdoor pond fish can be covered in fungus when the water temperature slowly increases from cold to warm.
  8. Marine fish fortunately rarely develop fungi.


The sooner you notice injuries on the fish and take action, the better the chances are that your fish will fully recover and its health will not deteriorate into a more critical state. The use of eSHa 2000 helps speed up the healing process of injured or damaged skin areas and prevent the development of mycosis, as well as combating fungi that have already developed. If mycosis has developed as a result of a protozoal infection, we recommend combining eSHa 2000 with eSHa EXIT for an even broader range of action.

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