How Do Goldfish Get Fungus on Them and Is It Harmful?

Some non-fungus diseases also make your fish look moldy, but once you know what to look for, fungus is unmistakable. True fungus disease looks like bunches of cotton with unmistakable hairy tendrils sticking out of them. Diseases commonly mistaken for fungus are Ichthyophthirius multifiliis ("ich" or "ick"), a protozoan parasite that looks like flat, white spots; and Flexibacter, a bacterial infection also called fin and mouth rot.

A dirty tank sets your fish up for fungus infection in two ways: First, the filthy conditions allow excessive amounts of fungus to grow. Second, ammonia from rotting fish waste and other organic matter wears away the slime coats that protect the fish, and cause chemical burns. These burns then get infected by fungus.

Overcrowding adds to the danger for your fish. More fish means more waste rotting in the tank, more bullying and fewer places to hide. All of these are stress factors, and stress reduces the ability of your fish to recover from wounds and fight off infections.

You've probably heard the "one fish per gallon" myth. This is simply not true. The bare minimum for a healthy aquarium is one gallon of water per inch of fish, and two gallons are better than one. The more space you have per inch of fish, the healthier the fish will be. Let's bust another myth: Goldfish growth is not limited by the size of the tank, so plan ahead.

Fungus can infect any wound, and unfortunately you probably will not be able to see wounds until they are infected. Improper or rough handling with hands, nets, or containers can wound your fish, as can rough or sharp decorations. Fish may get wounded fighting each other, or they may have open lesions from bacterial or parasite infections. All of these wounds are at risk for fungal colonization.

The best defense against fungus is a clean tank with lots of room for each fish. If your fish do get fungus, treat it immediately. Salt is the most widely recommended treatment, 1 tablespoon per gallon of water. It can be aquarium salt from the fish store, rock salt from the hardware store, or sea salt from the grocery store, as long as it isn't iodized. Other treatments include fish drugs malachite green, methylene blue, potassium permanganate, and fulvinex. Use these according to package instructions. They all will kill plants and snails, so if this is a concern, you should treat your sick fish in a separate tank. Some fish keepers report success with melaluca (tea tree oil) based fish medications, which do not kill plants or snails.