Columnaris (Cotton Wool Disease) in Betta Fish

Columnaris Cotton Wool Disease in Bettas
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Do you own a betta fish?

Then columnaris is something you’ll want to be aware of. This fatal bacteria can quickly kill off your beloved pet before you even have time to recognise the symptoms.

Columnaris is also called “mouth fungus” or “cotton wool disease” because one symptom of this disease is a cottony-white substance covering the upper lip and mouth area.

This can easily be mistaken for a fungal infection – in which the wrong treatment may be administered.

Due to the severity of this disease, you need to be able to recognise the symptoms in betta fish and take appropriate measures if columnaris are present.

In this post, I will explain what columnaris is, how it affects betta fish, and how it can be treated.

What is Columnaris?

Columnaris is a bacterial infection in betta fish caused by Flavobacterium columnare bacteria.

These gram-negative bacteria are named columnare due to their rod or column-shaped bodies.

When viewed under a microscope, Flavobacterium columnare bacteria show a cotton-like appearance.

The scientific name of columnaris is Flexibacter columnaris. The condition has several other names, like saddleback disease, and cottonmouth disease, but commonly known as cotton wool disease.

Many betta owners wrongly assume fungus is the cause of cotton wool disease, while in reality, betta fish rarely develop fungus in their lifecycle.

Freshwater fish, like bettas, are the primary target of columnaris. Columnare bacteria can kill almost any specie of freshwater fish (including betta fish) if left untreated for even a few days.

Columnaris is a highly contagious disease that quickly spreads from sick to healthy bettas.

Unlike many other conditions betta fish encounter, the cotton wool disease is often fatal if you don’t diagnose and treat it in its early stages.

Bettas with weak immune systems are likelier to die from this condition as they cannot fight columnare bacteria.

Modern research shows that columnaris may prove fatal even after treating it, especially when using ineffective treatment methods.

Flavobacterium columnare bacteria have two strains that can cause cotton wool disease. One of these strains is freakishly fast acting, while the other act slowly.

The fast-acting strain could kill the betta within only a day of infection, while the slow-acting strain takes a couple of days to finish off its target.

So, the strain of columnare bacteria infecting your bettas dictates the fatality of the columnaris condition.

What causes columnaris in betta fish?

It’s practically impossible to eliminate bacteria from your aquarium due to their microscopic nature.

Therefore, the cause of columnaris condition, Flavobacterium columnare, is often already present in your aquarium water.

So the factors that trigger the disease outbreak in the aquarium that weakens a betta’s immune systems are the real causes of columnaris.

Following are some important conditions that can trigger the spread of columnaris in your aquarium:

Poor water conditions or high bioload aquariums

If your aquarium has a heavy accumulation of bioload, it can increase the ammonia and nitrite levels in the water, which is harmful to your bettas.

That often occurs due to an inefficient filtration system, and betta fish living in aquariums with high levels of ammonia and nitrite can become stressed and susceptible to columnaris (and other diseases).

Small betta tanks are more likely to have high ammonia/nitrite/nitrate levels because of their lower volumes of water.

Overcrowding in the tank is another reason for bioload buildup because the more fish you have, the more waste they’ll produce.

The number of bettas in your tank should be proportionate to the aquarium size so that they can have sufficient space.

Consequently, poor water conditions weaken the immune system of betta fish.

Thus, following good aquarium maintenance practices is essential to maintain healthy water conditions in your aquarium.


Stress increases your bettas’ vulnerability to diseases.

They can become stressed due to many reasons, such as poor water conditions, sudden temperature changes, fighting with tankmates, and a low-quality diet.

Betta fish love to roam around in the aquarium and hide in dark places behind decorations.

So if you don’t have decorations in the aquarium, they may start feeling threatened and become stressed out.

In the same way other fish do, bettas’ immune systems will suffer if they remain stressed for long periods of time.

A betta’s resistance to columnaris disease decreases when the immune system is weakened.

The oxygen levels in the aquarium are low

Some blogs mention that if your aquarium has low oxygen levels, columnaris will thrive and infect your bettas.

I’m inclined to believe otherwise, as the betta’s natural habitat has low oxygen levels too.

That’s why bettas have labyrinth organs that help them breathe atmospheric air – so it doesn’t make sense to me that low oxygen levels would cause columnaris in bettas.

Instead, I believe it’ll affect non-labyrinth species as they absorb oxygen from the water column. With low oxygen levels, other species will feel stressed and succumb to columnaris.

Thus, if you’re keeping other fishes with your betta, you risk spreading it to your betta fish. But to claim low oxygen levels will cause betta fish to get columnaris is a bit of a stretch.

Low-quality diet

Bettas are very sensitive about their diet. As carnivores, they prefer an animal-derived (meaty) diet.

Carnivorous diets usually have a high content of fats and proteins. So take your time to compare different fish foods to see which is better for bettas.

I like using Dr Bassleer’s Biofish Garlic, especially when trying to coax wild bettas to eat pellet food, but other Dr Bassleer products are also great.

It’s also best to supplement them with live daphnia or frozen bloodworms. Check out my betta feeding guide on how to select your fish food.

Signs and symptoms of columnaris in bettas

Columnaris disease can be easily recognisable at all stages of its cycle.

According to other blogs, diagnosing cotton wool disease in betta fish is fairly easy by using a microscope to detect the presence of Flavobacterium columnare bacteria on its body.

But I don’t have a microscope, so I’ll share other ways to identify columnaris symptoms.

Skin itching and irritation

You may notice your bettas flicking or rubbing their bodies against a solid object (like decorations) or plants in the aquarium as a manifestation of columnaris.

They do so as a response to the discomfort they experience due to the disease. Sick bettas may also show other signs of parasitic infection, like skin itching.

However, both skin itching and irritation can also be Ich or Velvet.

You can distinguish between Ich/Velvet and Columnaris by checking the appearance of skin irritation; if it is white or gold spots, it’s Ich and Velvet, respectively.

If it’s fluffy or wool-like, it’s probably columnaris.

Damaged or frayed fins

If you notice damaged or frayed fins, it could be columnaris.

The cotton wool disease mainly affects them first and can cause fraying or raggedness in extreme cases, particularly around the edges.

However, if you notice fin rotting or frayed fins in your bettas, it could indicate fin rot, or your betta is biting its fins instead of columnaris.

Observing your betta for fin biting is the easiest way to eliminate columnaris. Again, check for a white wool substance on your betta’s fins to determine if it’s columnaris.

Fading body colouration

The cotton wool disease usually causes bettas to lose their body colouration.

But this is also another symptom of diseases in betta fish, so pair this with other symptoms to narrow down your problem.

Skin ulcers

Sick bettas may also develop ulcers or sores in certain areas of their skin.

Like fading body colouration, this is also a potential symptom of other diseases. So use this with other symptoms to identify columnaris.

Cotton-like growths

When cotton wool disease begins, colonies of Flavobacterium columnare bacteria form on the skin near the mouth and fins, resulting in a cotton-like appearance.

If you see these growths, chances are it is columnaris.

Although they are usually white, they may also have whitish-grey or yellowish-brown colours.

Gill necrosis

If the cotton wool disease is left untreated for a long, infected bettas will start showing colour changes in their gills.

Normally, betta fish have pink gills, but their colour can turn brown due to columnaris.

The colour change indicates the degradation or death of gills-surrounding skin. It also shows the onset of necrosis.

At this stage of columnaris, your bettas may also develop breathing issues.

Treating a betta with gill necrosis becomes extremely difficult.

If you try to treat them somehow, the chances of positive results are slim.

Gills, fins, and skin are betta’s body’s most commonly affected organs due to cotton wool disease.

It also happens sometimes that these organs develop a layer of slimy mucus produced by betta’s immune system as a response to the attack of Flavobacterium columnare.

So, looking out for a mucus layer can also be helpful in spotting columnaris.

If your bettas have developed swelling in their lips, it can also be a sign of cotton wool disease.

Columnaris-affected bettas may also stop eating due to their swollen mouth.

How to treat cotton wool disease in your bettas

It’s important that you start treating your betta immediately after spotting columnaris.

The disease can turn into a fatal condition if you leave it untreated.

You can follow these steps to treat columnaris in your betta effectively:

Quarantine your betta fish

The first step in treating cotton wool disease is shifting your bettas from the main aquarium to a separate quarantine tank.

Flavobacterium columnare bacteria thrive in warmer temperatures, so you’ll need to keep the water temperature of the quarantine tank slightly lower than the main aquarium to decrease their growth rate.

The optimum temperature for betta fish differs based on the species of betta, so make sure you set the temperature within the acceptable range.

Also, don’t drop temperatures immediately. Slowly decrease it by a degree every 30 minutes.

If you’re from the tropics and don’t have a chiller or don’t keep them in a room with air-conditioning, I would say it’s OK not to reduce the temperature.

If you want to do so, move it to a shady part of your house, use an aquarium fan, or just a normal fan to cool your aquarium.


After quarantining your bettas, the next step is to medicate them. As you’re treating bacterial infections, any recommended antibiotic medicine can be a good choice.

A wide range of aquarium antibiotics is available in pet stores to kill gram-negative bacteria, among which oxytetracycline and Maracyn-Two are the most common.

Oxytetracycline is easier to use because you can mix it in your betta’s food.

These medications might come with different brand names depending on where you’re from. So check the ingredient list or ask your local fish store what’s the best medication for columnaris.

Always remember to follow the manufacturer’s instructions when dosing medication.

Aquarium salt treatment

The aquarium salt treatment is an effective method to reduce stress levels in your bettas.

It also empowers their immune system and increases their ability to fight cotton wool disease.

You can directly mix the aquarium salt in the water of the quarantine tank as per the instructions given on the label of the packaging.

I have a guide on treating your betta fish with aquarium salt here, but I recommend following the manufacturer’s instructions and using my guide as a… guide.

Water changes

Water changes are vital in keeping the water conditions well-maintained in your aquarium.

Besides regular practice, you need to carry out a 25% water change every 2-3 days when treating your betta.

It will also reduce columnaris bacteria in your aquarium.

Preventing cotton wool disease in your bettas

Preventing the cotton wool disease from happening in the first place is way much better than treating it.

Following are some useful tips that can help keep your betta fish safe from columnaris:

  • Avoid overcrowding your aquarium as it can trigger stress in your bettas.
  • Keep your aquarium well-maintained and clean to avoid poor water conditions.
  • Carry out water changes every week. You can conduct a 25% water change of your aquarium every week.
  • If you have new fish that you want to add to the betta aquarium, keep them in a quarantine tank for at least two weeks to check for any signs of disease. If they don’t show any disease symptoms, you can put them into your tank after two weeks.
  • Quarantine newly bought plants too.
  • Despite being called fighting fish, betta fish love to live in a peaceful environment, so always choose non-violent tankmates for them.
  • Feed a balanced diet to your bettas.


In conclusion, columnaris is a serious condition that can be detrimental to the health of your betta fish.

Identifying the symptoms and treating the infection quickly and effectively is important to help your betta fish fully recover.

If you notice any signs or symptoms of the disease, start treatment immediately to keep the odds of recovery high.

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