Body fungus is among the more common goldfish ailments. It appears as fuzzy or slimy patches on the goldfish's skin. These can appear in several colors. While the disease can kill, you can usually save the fish. However, fungal infections usually indicate that something else has gone wrong in the aquarium and you must address this issue to prevent recurrence or complications.
Saprolegnia and related fungi cause the disease. These fungi live in almost all aquarium water, but usually do not cause infection. However, if adverse conditions weaken fish, these fungi can get a foothold and cause infection. This makes body fungus an opportunistic infection—an infection by an organsim that can cause illness only in weakened or injured organisms. In goldfish, poor water quality and overcrowding can set the stage for such infections.
An Ounce of Prevention
You need to address the underlying issue that left goldfish vulnerable to infection in the first place, or any treatment will be a stopgap measure at best. Poor water quality often comes down to overstocking. A goldfish needs at least 30 gallons of aquarium volume per fish to survive. While keeping a goldfish in less water than this may not immediately kill the fish, overcrowding leads to poor water quality which can stress goldfish. Stressed goldfish find themselves weakened and more prone to opportunistic infections like body fungus. Frequent water changes and a robust filtration system are also key to clean water and healthy fish.
Pet shops sell remedies to cure infections like body fungus. Many contain malachite green, a copper-bearing compound that treats a wide variety of fungus, bacteria and larger parasites. Always follow the manufacturer's instructions to the letter. Also, make sure you remove any chemical media from the aquarium, such as activated carbon inserts. These inserts can absorb medication, reducing their effectiveness.
With a Grain of Salt
In some situations, aquarium salt can cure body fungus. This typically only works in very early stages of the infection. Add 1 tablespoon of rock or aquarium salt per gallon of water. While this cure does not work every time, you can use it in conjunction with medications like malachite green.