What causes fin rot

Fish diseases: Mouth or fin rot (Columnaris disease)

Mouth or fin rot (Columnaris disease)

Fin rot

The Mouth or fin rot or "Columnaris disease" is sometimes called Mouth mold or Mouth rot designated.

The English name of this disease is Cotton woolDisease (Cotton sickness) or SaddlebackDisease (Saddleback disease).

In aquaculture, this disease is widespread among trout; in aquaristics, it mainly affects live-bearing species.

The is triggered Mouth or fin rot by the bacterium Flavobacterium psychrophilum (Synonym: Flexibacter columnaris). Mixed infections with other types of bacteria can also occur.

In most cases, this disease is caused by poor housing conditions or stress factors. Although the pathogens are usually found in almost every aquarium, they cannot harm healthy fish.


As one can see from the German name of this disease, milky white spots (similar to mold) form on the fins, on the edges of the scales and in the area of ​​the fish's mouth. The fish also appear apathetic and "stagger" through the water.

Will the Mouth or fin rot or "Columnaris disease" is not recognized and treated at an early stage, it can spread over the entire body of the fish: ulcers form, the lips of the fish are destroyed and the fins decompose down to the fin rays.

A distinction is made between a chronic and an acute form: in the chronic form, white areas slowly form and gradually become larger. If the affected fish are not treated, they will die. In the acute form, the white spots spread very quickly and the fish die within a very short time.

Preventive measures

  • correct water values, regular partial water changes
  • the stocking density should be low; newcomers should first be observed in a quarantine tank
  • good water filtration and ventilation
  • species-appropriate, varied diet for the fish

Biological treatment proposals

Will the Mouth or fin rot or "Columnaris disease" detected early, it can usually be treated successfully: frequent partial water changes until nitrite is no longer detectable. Also feed the fish varied and rich in vitamins. Avoid stress.

If this does not help or if the disease has progressed too far, it is essential to seek the advice of a qualified veterinarian.


The Mouth or fin rot is resistant to many different antibiotics (multi-resistant). It is therefore IMPERATIVE that a qualified veterinarian should be consulted.