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

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Goldfish can get a number of diseases in captivity. One common disease, and one of the most obvious, is fungus. While it is definitely something to be concerned about, the condition is not difficult to treat.


You can diagnose fungus visually. It will manifest as fuzzy patches on the skin of the affected goldfish. These patches vary in coloration. White, gray, green or brown patches may appear. However, all will appear fuzzy, like cotton. Infected fish may appear lethargic or refuse to eat.

What Is It?

A fungal infection of goldfish is called saprolegniasis. Several closely related fungi cause this disorder, including fungi of the genera Saprolegnia, Achlya and Dictyuchus. Species resemble each other so closely that mycologists, fungus scientists, have trouble telling them apart. Their causes, symptoms and treatments are the same, so fortunately it's not important which specific species caused the disease.


These fungi cannot infect a healthy goldfish. Usually, something must happen to a goldfish to make it vulnerable to fungal infection. An injury, a parasite or bad water conditions can cause such reduced immunity. For this reason, the fungus is called a secondary infection, an infection that took place after the fish was weakened by illness or injured. It's important to figure out what caused the initial problems with the goldfish, or the fungus could return after treatment.


Quarantine is important when treating this disorder, and can help prevent it. However, once the disease takes root, "malachite green" is the best treatment. Most pet shops sell fungus remedies that contain this ingredient. Scoop an infected fish with a cup -- don't net it -- and move it to a quarantine tank. Make a malachite green dip containing about 5 ppm malachite green; follow the manufacturer's instructions for exact dosage. Mix the dip solution in a bucket with an airstone. Once a day for a week, put the fish in this bucket for about 20 minutes. In the quaratine tank, add about 1 tablespoon of salt per 4 gallons of water to help the fish heal.