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What Is Columnaris in Goldfish?

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Columnaris is an infection that hits goldfish, as well as other pet fish. This infection has several very obvious signs and symptoms, which can be used to rapidly distinguish it from other goldfish illnesses. Colunaris infections can quickly kill a goldfish, so treatment must be rapid and address both the infection itself and the conditions that promoted it.

Naming Confusion

Columnaris has a very confusing set of names. It is commonly called "mouth fungus." However, a bacteria, not a fungus, causes this illness. The bacterial colonies look fuzzy, hence the nickname. The bacteria belongs to the species Flavobacterium columnare and was formerly classified as F. columnaris, so columnaris became another of the common names.


Columnaris infection presents in goldfish as a fuzzy, wool-like growth on the goldfish's mouth. The growth is usually off-white or gray, but it can appear brownish. It generally grows in patches. The infection will generally start on the mouth but can spread to the gills and the rest of the face as it progresses. Infected fish may appear lethargic and refuse to eat. Sometimes, the fungus can cause red ulceration on an infected fish's mouth and head.

Risk Factors

The bacterium that causes columnaris lives in most aquariums. However, it can cause infections only under certain circumstances. In order to harm a fish, columnaris has to be present in a poorly maintained aquarium. Goldfish are often kept in tanks that are too small for them, leading to poor health. A goldfish needs at least 30 gallons to thrive. Goldfish kept in smaller tanks and bowls tend to foul the water quickly and are more prone to infections like columnaris.


In addition to improving aquarium conditions, quick action is required to save the goldfish. Columnaris bacteria are susceptible to gram-negative antibiotics like kanmycin. In the United States, you can purchase aquarium antibiotics at pet shops, but in the United Kingdom, you may need a vet's prescription to pick up antibiotics for aquarium fish. It's a common belief that the addition of salt can help cure columnaris; unfortunately, the amount of salt required would probably harm a goldfish.