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Columnaris - Flexibacter columnaris



  • Names: Columnaris, Cotton-Wool, Cotton-Mouth, False Neon Disease, Flavobacterium Columnare, Flexibacter, Guppy Disease, Mouth Fungus, Mouth-Rot, Saddle Back
  • Disease Type: Bacterial (gram negative rods)
  • Cause / Organism: Flexibacter columnaris


Often mistaken for a fungal infection because of its mold-like lesions, Columnaris is a common bacterial infection in aquarium fish, particularly livebearing fish and catfish. Its name is derived from columnar shaped bacteria, which are present in virtually all aquarium environments.

The columnaris bacteria are most likely to infect fish that have been stressed by conditions such as poor water quality, inadequate diet, or stress from handling and shipping. Columnaris can enter the fish through the gills, mouth, or via small wounds on the skin. The disease is highly contagious and may be spread through contaminated nets, specimen containers, and even food. For this reason it is important to use sterile techniques to avoid contaminating other tanks. Prophylactic treatment of all other tanks is wise, and is mandatory if they share a common filtration system.

Columnaris can be external or internal and may follow a chronic or acute course. Lesions in chronic cases progress slowly, taking many days before culminating in fish death. In acute cases the lesions spread quickly, often wiping out entire populations of fish within hours. High water temperature accelerate the progression of the disease; however lowering the water temp will not affect the outcome of the disease.


  • 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
  • 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. Gills as 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.


  • Change water
  • Vacuum gravel
  • Add aquarium salt
  • Treat with copper sulfate or antibiotic
  • Discontinue carbon filtration during treatment
External infections should be treated with either 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 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
  • 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 fish, and promptly moing 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 livebearer aquariums. When fish are being shipped or moved they are under stress, which leaves them open to contracting disease. To give them a better chance of remaining healthy they may be give prophylactic antibiotic treatment, or fed medicated food.

Readers Respond: How Aquarium Owners Cope With Columnaris

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