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Columnaris Disease

(Mouth Rot/Fungus, Fin Rot)

Affected fish

All species, particularly Poecilia sp. such as Guppies and Mollies, weather loaches (Misgurnus sp.), and loaches


The bacteria Flexibacter columnaris is the pathogen for this disease, and it can be found readily in aquariums or carried by otherwise healthy aquarium fish. Outbreaks occur often as a result of temperature fluctuations, excessive organic waste, or in particularly prone species. The bacteria are most common in hard, alkaline water.


A white or greyish patch often appears on the body, followed by erosion surrounded by a reddened area and inflammation. Loss of colour is typical and rapid breathing or fin rot may also be symptoms. In livebearers and other fish, swimming on the spot motion, often referred to as shimmying is a typical symptom. When the bacteria heavily infects the mouth area, often seen in goldfish and carp, the lower portion of the jaw is eaten away, giving typical symptoms of mouth rot.


Avoid any build-ups of organic matter (such as leftover food) and temperature fluctuations.


Anti-bacterial treatments will help, but some will be more effective than others. Copper-based treatments can also be effective. Antibiotics are best, although these are only obtainable from a vet.

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