Fish fungus 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. Saprolegnia normally establishes as small, focal infections that then spread rapidly over the body or gills. As it spreads, healthy tissue is destroyed. There is often little inflammation unless there is an underlying bacterial infection. Microscopic examination shows broad, non-septate (no dividing cell walls) hyphae of varying diameters.
Cause: Saprolegniasis is mainly a secondary infection seen after damage to the fish integument (skin and gills) caused by parasites, viruses, bacterial infections and other skin damage. Other predisposing factors include water pollution and overcrowding. Less commonly, Saprolegnia can act as a primary pathogen infecting fish that haven’t shown signs of previous damage. It is believed that such attacks are temperature-dependant, usually occurring at low temperatures, possibly as a consequence of a reduced immune response. I have seen two separate cases recently involving orfe that were severely infected without any other signs of damage.
Treatment:
Fish fungal infections are difficult to treat and mould cannot ever be eliminated from any fish keeping systems. Any treatment plan must take account of any predisposing factors and these should be resolved / or treated at the same time.
Use strong malachite green solution (100mg/ litre) to clean the lesion and apply a water-proof cream.
Bath: Salt 1–5g / litre indefinitely, Malachite green: 0.10mg/litre – three treatments at three-day intervals
