Fish fungus is a fluffy white or grey growth caused by Saprolegnia spores. The spores are present in most freshwater aquariums in small numbers, but healthy fish do not become infected because their protective slime layer provides a barrier against a broad spectrum of aquarium bacteria. However, if a fish is injured and a wound is left open to infection in dirty water, fish fungus can develop. Live bearers are especially susceptible to Saprolegnia, and a preventive dose of aquarium salt, in accordance with the manufacturer's instructions, can offer protection to guppies, mollies and platies. Cure infected fish with a commercial anti-fungus treatment.
Diagnose your fish correctly before you start treatment. Fish fungus looks like a fluffy cotton or wool growth around the gills, fins or mouth, and is caused by Saprolegnia. Mouth rot, or cotton mouth, is often present at the same time as fish fungus and looks very similar, but it is caused by the Flexibacter or Chondrococcus Columnaris bacteria. You can treat both diseases with the same medication.
Treat your aquarium with a broad spectrum commercial anti-fungus medication to eradicate all spores. Several brands are available, but they all do the same job and kill a wide variety of fungal spores, preventing the disease from spreading to other fish in the tank.
Follow the anti-fungus medication instructions carefully, and continue treatment for the recommended length of time, even if your fish appear to be recovering. Active ingredients are methylene blue or phenoxethol (see warnings), and different brands of the same medication may have very different dosage instructions, so read the instructions and administer the correct dosage with care.
Carry out a partial water change at the end of a successful treatment cycle because fish medication remains in the water. Unnecessary latent chemicals in your aquarium may cause irritation and harm your fish, particularly when they are still vulnerable from a recent bout of fish fungus.
Add aquarium salt, following the dosage instructions on the box, as a preventive measure so that the fish fungus does not return to your tank.
Keep your aquarium water clean and healthy at all times by carrying out regular partial water changes, and do not overfeed your fish.
Anti-fungus medications containing methylene blue will turn your aquarium water blue. Phenoxethol is an alternative ingredient. It's equally effective, but don't use it frequently. Methylene blue may stain aquarium ornaments, silicone sealant, your skin and fabrics.