Goldfish fungus disease comes in two forms, the more commonly seen skin form, and the much more dangerous gill form.
Saprolegnia fungus disease, also known as cotton wool disease is one of the more common goldfish diseases. Fungus doesn’t affect healthy fish. If a fish has sustained an injury, this can give fungus the opportunity to invade the area.
Prolonged low water temperatures may make fish prone to fungus attack as it often occurs in early spring, late autumn, and winter.
Fungus can also be a symptom of parasites as fungus spores are able to penetrate damaged skin or gill membranes. If parasites are present, usually flukes or a protozoan parasite such as Costia, the fish needs to be treated for these at the same time as the fungus.
A fish that has a weak immune system due to a poor environment will also contract fungus.
Fungus appears as patches of grey/white cotton wool on the body or fins. If it is left untreated it will spread from the skin into the muscle tissue and the fish will eventually die.
is an aggressive fungus that kills fish by destroying gill
tissue. The affected areas die and slough off leaving gaping
holes in the gill membrane. Eventually the fish suffocates.
To flourish Branchiomyces requires high water temperatures, high levels of waste in the water, high nitrate levels, and overcrowded conditions.
Summer is the usual
time for this fungus type to appear because of higher water temperatures
and more frequent feeding.
There are several treatments that can be used for Saprolegnia. The Goldfish should already be in a mild salt bath:
Some of the chemicals mentioned above are fairly toxic.
I gradually increase the Aquarium salt content to 0.3% over a 24 hour period (3-4 grams per liter or 1 tablespoon per US gallon). I then add Methylene Blue until the water is medium blue. Goldfish can tolerate quite high doses of these chemicals safely. The cure is slower but safer.
Depending on water temperature and medication level, it can take a week or more before the fungus drops off the affected area.
Feed live foods if available to speed up recovery.
Keep the fish in the sickbay until all traces of fungus have disappeared and the affected area has started to heal.
For Branchiomyces use 2 milliliters of methanol free formalin per 10 US gallons (50PPM). Dose the fish for exactly 2 hours, then do a 75% water change. Observe the fish closely during this period. If it starts gasping at the surface stop treatment immediately.
After it has recovered, or if you have any doubts, use half the dose.
After treatment gradually add Aquarium salt to make a 0.3% salt bath and leave the fish in this until it is active and eating.
Warning: Formalin takes oxygen from the water, so don't use it in water above 80oF (27oC), and provide aeration in the sickbay during treatment.