Columnaris Disease in Aquarium Fish

Causes, Treatment, and Prevention

Platy fish
Mark Dumont / Flickr / CC By 2.0

Columnaris is a bacterial infection that can be external or internal and may follow a chronic or acute course in aquarium fish. Often mistaken for a fungal infection because of its mold-like lesions, the columnaris bacteria (Flavobacterium columnare) can be treated with antibiotics and prevented with basic tank maintenance.

What Is Columnaris?

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, though it has also been referred to as cotton wool disease, saddleback disease, guppy disease, or cotton mouth disease.

Symptoms of Columnaris in Fish

Lesions in chronic cases generally progress slowly, taking many days before culminating in fish death. In acute cases, the lesions can spread quickly, often wiping out entire populations of fish within hours.


  • White or grayish spots or patches on the head, fins, or gills
  • Lesions on the back that may extend down the sides
  • Moldy or cottony-looking lesions on the mouth
  • Frayed fins

Most columnaris infections are external and present first as white or grayish spots or patches 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; 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 which is often used to describe this symptom. On the mouth, the lesions may look moldy or cottony, and the mouth will eventually become corroded. The fins will erode and have a frayed appearance as the infection progresses. The gills can be affected, too; as the bacteria invade them, the filaments will disintegrate, resulting in rapid breathing or gasping due to lack of oxygen absorption. Less commonly, the infection will take an internal course with no external symptoms. In these cases, only necropsy and bacterial cultures will point to the true cause of death.

Causes of Columnaris

The columnaris bacteria are most likely to infect fish that have been stressed by conditions such as:

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 necessary if they share a common filtration system.​


External infections should be treated with antibiotics, chemicals in the water, or both. Copper sulfate, Acriflavine, Furan, and Terramycin may all be used in the water 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 (1 to 3 teaspoons per gallon of water) to reduce osmotic stress on fish from the damage to the fish's epithelium caused by the bacteria. 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, err on the side of caution when using salt.

How to Prevent Columnaris

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 into quarantine, 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. Commercial preparations of Benzalkonium Chloride solution (Net Dip or Net Soak) are available to use on nets and other items, or the items can also be soaked in a solution of 3% hydrogen peroxide.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.
Article Sources
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  1. Declercq, Annelies Maria et al. Columnaris Disease In Fish: A Review With Emphasis On Bacterium-Host InteractionsVeterinary Research, vol 44, no. 1, 2013, p. 27. Springer Science And Business Media LLC, doi:10.1186/1297-9716-44-27

  2. Bacterial Diseases Of Fish - Exotic And Laboratory Animals - Veterinary ManualVeterinary Manual.