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Fungal Infections (Saprolegnia)


Fungal infections as we know it in koi and koi ponds are caused by different groups of organisms. These organisms include true Fungi and water moulds.  Perhaps the most common “fungal” infection is Saprolegnia, which is actually classified as a water mould (Oomycetes) and not a true fungus.  Moulds reproduce by releasing thousands of spores into the surrounding water and are therefore ever present, regardless of how sophisticated the filter systems are. The fungal spores are highly resistant to drying and chemical attack and thus Saprolegnia spores and moulds are common in all ponds and tanks. This mould in short time will cover a dead insect or uneaten food that is left in the filter. Oomycetes therefore forms part of the process in the filter system that breaks down trapped waste.

It is believed that the fish mucus contains effective fungicides that prevent fungal growths under normal circumstances. Fungal diseases in koi are usually external and almost always a secondary opportunistic infection that sets in after injury, trauma, parasite activity, poor water quality, viruses, bacterial infections and other skin damage. These fungal growths usually start as small, focal infections with the fungus feeding on dead tissue associated with the small wound on the koi and eventually spreading over more and more of the koi's body. Saprolegnia is a common fish disease that can prove fatal if not treated early. In common with all moulds, Saprolegnia feeds by secreting digestive enzymes onto the surrounding area. These enzymes break down cells and tissues enabling the fungi to absorb nutrients such as proteins and carbohydrates. In fact your fish is being slowly digested!

This photo demonstrates a chemical burn through accidental application of the wrong topical treatment, followed by a Saprolegnia infection.


Saprolegnia infection appears as grey/white patches on the skin or gills that resemble tufts of cotton wool. At a later stage they may become brown or green as they trap sediment or algae. If the fish is removed from the water, the fungus appears as a slimy matted mass.


Fish fungal infections are difficult to treat, as mould cannot ever be eliminated from any fish keeping systems. Any treatment plan must take into consideration that repeated topical applications would be necessary. The cause of the infection should also be resolved/or treated at the same time. The chances of recovery from Saprolegnia infection are directly related to the area affected. As the infected area can rapidly increase, prompt diagnosis and treatment is vital. There is sometimes little inflammation unless there is an underlying bacterial infection. During treatment one must always assume that a bacterial infection is present or at least prevalent and a course of antibiotic injections are recommended.

       fungus_showa   fungus_showa_2

Topical treatments

Clean the lesion by wiping it with cotton wool dipped in 100mg/litre Malachite Green solution, 100mg/litre Methylene Blue solution, 1 gram/litre of Potassium Permanganate or 3% volume Hydrogen peroxide. Dab the area dry and seal the lesion with Bao Bio’s Woundmed (it contains Malachite Green) and a sealant.



Alternatively the wound can be dabbed dry and then sealed with Propolis or Friars Balsam.

It is important to treat and seal the wound at least every three days to prevent Saprolegnia from taking hold again.


A series of salt or Potassium Permanganate dips will also get rid of the mould, provided the lesions are sealed regularly.

Pond treatments

Salt at 0.3% in pond until cured.

Potassium Permanganate at 2ppm, three treatments, three days apart.

Malachite green at 0.10mg/litre, three treatments, three days apart.

Last Updated on Monday, 06 December 2010 15:28