Aquarium Fish Health: Dealing With Cotton Mouth Disease (Mouth Fungus)

Aquarium Fish Health: Dealing With Cotton Mouth Disease (Mouth Fungus)

Written By: text_none_author Published In: Fish-Blog Created Date: 2016-10-13 Hits: 2963 Comment: 0

Cottonmouth (also referred to as Mouth Fungus and Columnaris) is a symptom of disease in fish which results from an infection caused by the Gram-negative, aerobic, rod-shaped bacterium Flavobacterium columnare. It was previously known as Bacillus columnaris, Chondrococcus columnaris, Cytophaga columnaris and Flexibacter columnaris. The bacteria are ubiquitous in fresh water, and cultured fish reared in ponds or raceways are the primary concern – with disease most prevalent in air temperatures above 12–14 °C. It is often mistaken for a fungal infection. The disease is highly contagious and the outcome is often fatal. It is not zoonotic. Cotton Mouth disease is not as common as the while spot disease, but, it is highly infectious and contagious.


  • White spots on mouth, edges of scales, and fins,
  • Cottony growth that eats away at the mouth,
  • Fins disintegrate beginning at the edges,
  • 'Saddleback' lesion near the dorsal fin,
  • Fungus often invades the affected skin, and
  • Rapid gilling in cases where gills are infected.

Most Columnaris infections are external, and present first as white or grayish white spots on the head and around the fins or gills. The lesions may first be seen only as a paler area that lacks the normal shiny appearance of the rest of the fish. As the lesion progresses it may become yellowish or brownish in color and the area around it may be tinged red.

Lesions on the back often extend down the sides, giving the appearance of a saddle, leading to the name saddle-back that is often used to describe this symptom. On the mouth, the lesions may look moldy or cottony, and the mouth will eventually become eaten away. The fins will erode and have a frayed appearance as the infection progresses. The gills are affected, too. As the bacteria invade them the filaments will disintegrate, resulting in the onset of rapid breathing or gasping in the fish due to lack of oxygen. Less commonly, the infection will take an internal course which often displays no external symptoms. In these cases, only a necropsy and cultures will point to the true cause of death.


Unless the affected fish is of consideration value, it should be killed before this fatal disease attacks the other occupants, of the tank. Think about it... is trying to save the life of one fish worth risking the death of the rest of the fish in your aquarium? 

But if you insist on keeping the fish or in case the infection has already been passed on to other occupants, the following treatment is advised: 

  • Change water,
  • Vacuum gravel,
  • Add aquarium salt,
  • Treat with copper sulfate or antibiotic, and
  • Discontinue carbon filtration during treatment.

External infections should be treated with antibiotics, chemicals in the water or both. Copper sulfate, Acriflavine, Furan, and Terramycin may all be used externally to treat Columnaris. Terramycin has proven to be quite effective both as a bath, and when used to treat foods for internal infections. Salt may be added to the water to enhance gill function. Livebearers, in particular, will benefit from the addition of salt; however, use caution when treating catfish, as many are highly sensitive to salt. When in doubt, check error on the side of caution when using salt.


  • Quarantine new fish for two weeks,
  • Maintain high water quality,
  • Provide fish with a nutritionally balanced diet,
  • Medicate fish prophylactically before moving them, and
  • Disinfect nets and other equipment before using.

Because the bacteria thrive on organic wastes, the potential for Columnaris outbreaks can be controlled by regular water changes and tank maintenance, including vacuuming of the gravel. Proper diet and maintaining good water quality in general will keep the fish from being stressed and therefore more susceptible to infection. Placing new fishes and promptly moving any sick fish to a quarantine tank will help prevent the introduction and spread of the disease.

To avoid spreading the bacterium to other tanks, nets, specimen containers, and other aquarium equipment should be disinfected before each use. Small quantities of aquarium salt can be used regularly as a prophylaxis to prevent disease in live-bearer aquariums. When fish are being shipped or moved they are under stress, which leaves them open to contracting diseases. To give them a better chance of remaining healthy they may be given prophylactic antibiotic treatment or fed medicated food.

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