Koi fungus, or Saprolegnia, grows in cotton wool looking patches both on the scales and sometimes in the gills of a Koi. The fungus, if left untreated, can kill the fish and is most commonly contracted by injured or stressed Koi or by Koi kept in dirty, poorly filtered water.
According to Koi-Planet.com, Koi fungus appears as a cotton wool-like growth that is often fuzzy at first but that will grow into longer threadlike filaments hanging from the Koi as time progresses. The fungus may sometimes appear to have a mucus-green tinge which is the cause of algae growing in the fungus.
Koi fungus spores will often grow around cuts or dead tissue on the Koi. The fungus secretes an enzyme which it uses to break down a cell and absorb the nutrients. The most common breeding grounds for the fungus are dirty or overcrowded living conditions.
The fungus may start out in patches but can advance to cover a good portion of the Koi's body. According to Koi-Planet.com, while the fish can die from the spreading fungus, they more often succumb to the wound or infection that created the open sore or dead tissue in the first place.
Sometimes salt baths may be effective, but a common solution to combat Koi fungus is to treat the fish with Malachite Green, an extremely powerful disinfectant used to cure a wide number of infections in fish, or another similar product. The wound on which the fungus germinated must be treated and properly cleaned as well.
According to the Olympic Koi, Goldfish and Water Garden Club, a Koi pond should be at least three feet deep and hold no less than 300 gallons of water. While a number of materials can be used to make the pond, the Koi will do best, and be more likely to avoid fungal infections, when kept in properly filtered and aerated waters. A Koi will usually get to around two feet in length and a healthy fish can live to be 30 years old.