Fungal attacks always follow some other health problem like parasitic attack, injury, or bacterial infection. The symptoms are a gray or whitish growth in and on the skin and/or fins of the fish. Eventually, if left untreated, these growths will become cottony looking. The fungus, if left untreated, will eventually eat away on the fish until it finally dies.

Fungus (Saprolegnia)
Symptoms:
Tufts of dirty, cotton-like growth on the skin, can cover large areas of the fish, fish eggs turn white.

After ascertaining the initial cause of the fungus and remedying that, use a solution of phenoxethol at 1% in distilled water. Add 10 ml of this solution per liter of aquarium water. Repeat after a few days if needed, but only once more as three treatments could be dangerous to aquarium inhabitants. If the symptoms are severe the fish can be removed from the aquarium and swabbed with a cloth that has been treated with small amounts of povidone iodine or mercurochrome.

For attacks on fish eggs, most breeders will use a solution of methylene blue adding 3 to 5 mg/l as a preventative measure after the eggs are laid.

Ichthyosporidium
Symptoms: Sluggishness, loss of balance, hollow belly, external cysts and sores.

Ichthyosporidium is a fungus, but it manifests itself internally. It primarily attacks the liver and kidneys, but it spreads everywhere else. The symptoms vary. The fish may become sluggish, lose balance, show hollow bellies, and eventually show external cysts or sores. By then it is usually too late for the fish.

Treatment is difficult. Phenoxethol added to food as a 1% solution may be effective. Chloromycetin added to the food has also been effective. But both of these treatments, if not watched with caution, could pose a risk to your fish. It is best, if diagnosed soon enough, to destroy the affected fish before the disease can spread.