Columnaris is a relatively common, yet potentially deadly bacterial infection that can affect both saltwater and freshwater fish.

It can cause a range of symptoms, from white patches on the body to outright death, so it’s important to be able to identify it and take action quickly if your fish develops any of the telltale signs.

In this article, we’ll take a closer look at what columnaris is, how to prevent it, and how to treat an infection if it does occur.

So without further ado, let’s jump into the details.

What is Columnaris?

Columnaris is the common name for a disease called Flavobacterium columnare, which is also commonly referred to as “mouth fungus” or “cotton wool.”

It was formerly called Bacillus columnaris, Chondrococcus columnaris, Cytophaga columnaris, and Flexibacter columnaris.

This disease is caused by a parasite known as Acantelasma denticulatum that attacks the fish’s mouth and gills, often resulting in sores that grow larger and more numerous until they reach the outer layer of skin.

It’s frequently confused with a fungal infection, however, this infection is highly transmissible, and the outcome is usually fatal.

Despite the fact that it was initially developed in the 1940s when antibiotics were not readily available to amateurs, it continues to inflict significant losses on our hobby today.


a. Physical Signs

At first, columnar infection shows up as white or grayish areas or patches on the head and around the fins or gills.

It can also just be recognized as a paler region that lacks the usual glisten of the rest of the fish.

As the lesion progresses, it may take on a yellowish or brownish hue as well as a pink tinge around it.

Signs Appear on the Body Surface

Small lesions develop at the base of the pectoral or dorsal fins as tiny, pale patches of discoloration.

As time goes on, these regions grow in size and cover up to 20–25% of the fish’s total surface area.

When the skin on the back is injured, it frequently spreads down the limbs, resulting in a saddle-back appearance,  giving the symptom the name “saddle-back.”

The surface of these wounds has a slight lemon-yellow tinge, and the skin peels away entirely.

Moreover, there are a large number of germs at the advancing margins of the lesion.

Signs Appear on the Mouth

The mouth lesion may appear moldy or cottony, and the mouth will eventually be eaten away.

Signs Appear on the Fins

The fins will begin to erode and fray as the infection advances.

b. Behavior Signs

When the gills are affected, as the bacteria spread throughout them, the filaments will break down, resulting in rapid breathing or gasping because of a lack of oxygen absorption.

Loss of appetite and sluggish swimming are also common symptoms.

How fast does this disease spread?

In chronic infections, the wounds develop slowly and can last for several days before resulting in fish death.

However, in acute situations, the injuries quickly expand, often wiping out entire species of fish within hours.

It’s crucial to note that elevated water temperatures speed up the disease’s progression; however, lowering the water temperature will not influence its outcome.


Columnaris infects fish thru their gills, mouths, or small wounds and is common in areas with high bio loads or stressful conditions such as overcrowding or low dissolved oxygen levels in the water column.

When hardness is 50 ppm or more, this microbe may survive in water for up to 32 days.

That means it’s highly transmissible via unclean nets, specimen containers, and even food since it can stay in water for that long.

Because of this, it’s critical to utilize sterile procedures in order not to pollute other tanks.

There are some causes of this disease:

  • Poor water quality
  • There is not enough aeration in the aquarium.
  • Sudden changes in temperature or pH
  • Overstocking
  • The introduction of a new fish that is a carrier of the illness to an already healthy aquarium
  • It can occur as a secondary infection from other diseases, such as fin rot, Velvet (gold dust disease), or excessive aggressiveness.


1. When you first detect symptoms of Columnaris, move the infected fish to a different aquarium so that it does not infect any other fish.

2. Then, use an antibiotic solution to destroy the bacteria that are causing the illness in the tank.

Columnaris can also be treated with copper sulfate, acriflavine, furan, and Terramycin.

3. To relieve osmotic pressure on fish as a result of the bacteria’s tissue damage, add salt to the water (1 to 3 teaspoons per gallon).

Livebearers, in particular, will benefit from the addition of salt.

4. Lower the aquarium temperature to 75 °F (24 °C). This step is essential since columnaris is more hazardous at higher temperatures, especially 85–90 °F.

5. Increase oxygen in order to improve their respiratory efficiency.

6. Providing your fish with medicated food containing oxytetracycline is also beneficial in the treatment of internal disorders since it will help in the treatment of the illness.

Note: Before treatment can begin, the tank must have a culture of live bacteria to ensure that the medication will function properly.


1. Change Water

Since the germs thrive on organic waste, columnaris infections can be prevented by water changes and tank maintenance, including gravel vacuuming.

Change 20-30% of your filter media every week (or more if it becomes dirty) to reduce the number of organic compounds that will eventually break down.

In this way, you can reduce the possible causes of columnaris infection.

2. Reduce Stress

The disease usually develops as a symptom of stress due to transport, a new tank mate, or overfeeding.

Check your aquarium’s conditions so Columnaris does not have the opportunity to develop.

3. Quarantine the new fish

Before they are introduced to your aquarium, quarantine all new fish for two weeks in a separate container (one per species) with only dechlorinated water and no filtration or aeration.

If the individual fish appears sick, contact your local aquarium store for additional information regarding medications and treatment methods.

Be sure to observe all quarantine treatments carefully since it is critical to prevent the spread of Columnaris within an aquarium or among multiple aquariums.

4. Provide your fish with a good diet.

If fish are given a balanced diet from an early age, they will be more resistant to illness, and if they become ill, it’s easier to treat them.

In order to provide your fish with a good diet, feed them a few times per day with small portions of food.

5. Remove any dead or dying plants.

The bacteria that causes columnaris may be present on dead or dying plant leaves.

If you see your plants beginning to show symptoms of disease, remove them immediately in order to prevent the spread of this bacterium in your aquarium.

Antibiotics for Columnaris

Columnaris is a bacterial disease that can have various manifestations in fish.

There are several effective antibiotics that can combat columnaris, such as Furan (Furan 2); Terramycin (Aquarium Products Spectrogram); or Tetracycline (Aquatronics General Tonic).

However, it is important to know that antibiotics cannot cure Columnaris 100% of the time, and sometimes they will not work at all.

So it is important to maintain good water quality during treatment since the presence of organic material in the tank can actually worsen a fish’s condition.

In fact, antibiotics might even be counter-productive if they are combined with high doses of organic material, such as excessive food in the tank.

In addition, antibiotics should be used with care since some of them can damage biological filtration if the dosage is too high.

For this reason, it is not recommended to remove the carbon from your filter when using medications containing Terramycin or tetracycline since these treatments are very strong chemicals that can be harmful to other tank life, including nitrifying bacteria.

If the medication is not effective at controlling Columnaris, you can try adding salt to your tank (high-quality aquarium salt) in order to reduce the effects of the infection and prevent secondary infections caused by Columnaris.

It should also be noted that if you add salt to your tank, you need to ensure your fish can tolerate it (i.e., by doing water tests).

Note: All medications have detailed instructions listed on their label, so it is important for you to read them carefully.

Last Words

Columnaris is a disease that affects many different types of fish, and it can be fatal.

Therefore, it’s important to keep your tank as clean as possible in order to avoid catching the infection. 

We hope you enjoyed this article as much as we did.

If you still have any questions, please share them with us in the comment section below.