COTTON WOOL DISEASE
a fungal disease caused by Saprolegnia sp. and Achyla sp. Appears as fluffy, whitish growths often at the site of an injury or diseased part of a fish. The fungi may turn grey to reddish-brown over time owing to dirt or algal accumulation. Can prove fatal if untreated as it will spread to healthy tissue. Occurs, for example, on salmonid gills and mouth, and on fins it is called fin rot. May also attack fish eggs, especially those that are infertile or damaged, but it may spread to healthy eggs (parental care fishes usually pick out infected eggs and broadcasts spawners usually have the eggs widely dispersed). Stress, chilling, old age and poor aquarium husbandry all contribute to this disease. Mouth fungus and columnaris disease (q.v.) have a similar appearance and may be called cotton wool disease but are bacterial diseases