Furunculosis in Betta Fish: Symptoms, Treatment, Prevention

Furunculosis in betta fish symptoms treatment prevention
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Furunculosis is an infectious disease commonly affecting freshwater fish species, such as betta fish.

It’s a contagious health condition that spreads from sick to healthy betta fish, making it a significant concern for betta owners.

However, with the proper knowledge about its causes, symptoms, and treatments, you can take better care of your bettas against this potentially fatal ailment.

Here is a guide to the causes and symptoms of furunculosis in betta fish.

You will also learn about how to treat this disease and what measures you can take to prevent it from happening in your bettas in the first place.

What is furunculosis betta?

Furunculosis is a bacterial infection in betta fish caused by a bacterium called Aeromonas salmonicida. 

This bacterium can be found in almost all aquatic environments and thrives at temperatures above 10˚C.

Furunculosis mostly targets but is not limited to fish species living in freshwater.

On the onset of the infection, a small, red sore or lesion appears on the fish’s body, which gradually grows in size with the progression of the disease.

The sore or lesion may become inflamed if the infection is severe.

As a highly contagious disease, furunculosis can pass on from sick to healthy fish through horizontal transmission via the water column or direct contact.

Bacterial infection may also be transmitted through the wastes of the affected fish.

What causes furunculosis in betta fish?

As mentioned earlier, the primary cause of furunculosis is the bacterium Aeromonas salmonicida.

Anything that causes the weakening of the immune system in betta fish and makes them susceptible to developing bacterium infection may also be considered the cause of furunculosis.

Here are some common causes of furunculosis in betta fish:

  • Physical injury or trauma

The bacterium that causes furunculosis can enter the betta’s body through physical injuries such as wounds or cuts and can cause infection.

Injuries on the skin, fins, or gills are easily accessible entry points for bacteria into the fish’s body.

  • Poor water conditions

Clean and well-maintained water conditions are essential for your betta’s health and well-being.

Suppose the water in the aquarium has an excessive buildup of wastes, higher levels of ammonia and nitrates, and food leftovers.

In that case, it can create a favourable environment for bacteria where they can thrive and cause outbreaks of furunculosis.

Such an environment also weakens the immune system of betta fish, making them susceptible to affective from bacterial infection.

  • Malnutrition

Betta fish require a balanced and nutritious diet containing both fatty and meaty contents for optimum growth.

If fed an unbalanced, low-quality diet, they may develop malnutrition, which weakens their immune system and makes them vulnerable to bacterial infections like furunculosis.

  • Stress

As a sensitive fish species, bettas can become stressed by minor changes in their living environment or routine.

Stress can weaken their immune system and affect their ability to fight infections.

Some common factors that can trigger stress in your bettas include:

  • Sudden changes in water temperature
  • Changes in pH level
  • Aggressive tank mates
  • Lack of decoration and live plants in the aquarium that bettas can use as hiding places
  • Overcrowding

Overcrowding your aquarium increases the risk of physical injuries and traumas in your betta fish, making them vulnerable to furunculosis.

When there are too many fish in a small fish tank, it creates a stressful living environment for your bettas, weakening their immune system and capability to fight off infections.

  • Poor aquarium maintenance

Proper aquarium maintenance is crucial to keeping your betta fish’s environment clean and healthy.

If the aquarium is poorly maintained, it can create a thriving environment for bacteria that cause furunculosis.

The equipment in the fish tank, such filtration system or heaters, should be adequately maintained and replaced when needed to prevent the growth of harmful, infection-causing bacteria.

Symptoms of furunculosis in betta fish

Furunculosis is a bacterial infection that accounts for a high mortality rate in betta fish.

The symptoms may appear within a few days of the infection and lead to fish death unless effective treatment is applied.

Following are some significant symptoms you may notice in your betta affected with furunculosis:

  • Sores and lesions on the skin

One of the primary and most noticeable symptoms of furunculosis is the presence of red, swollen, and painful sores or lesions on the fish’s skin.

The sores may be small initially but can grow in size with the progression of the infection.

In severe cases, betta fish may develop haemorrhages on their skin, mouth, and fin bases.

These haemorrhages may spread to muscles and internal organs, interrupting normal body functions.

  • Lethargic behaviour

Betta fish suffering from furunculosis behave lethargically, showing physical inactivity and abnormal swimming patterns.

Bacterial infection lowers their energy level, making them dizzy and fatigued.

In such conditions, betta fish spend most of their time resting at the bottom of the aquarium or sticking to a plant or decoration.

  • Loss of appetite

Fish with furunculosis may lose their appetite partially. They may cease eating altogether if the infection is left untreated for long.

If the loss of appetite prolongs in the fish, they may start losing weight or even become emaciated.

Emaciation in betta fish can cause weakness in the immune system, making them prone to developing bacterial infections.

  • Changes in body colour

Furunculosis may cause the body colour of betta fish to darken. You may also notice pale gills.

The fish may completely lose their body colouration if the infection is severe.

  • Swimming difficulty

As the infection progresses, infected betta fish may start experiencing difficulty swimming, either because of painful sores or lesions on their skin or due to haemorrhages in their muscles which cause weakness.

You may notice them swimming upside down on the water’s surface in response to the difficulty they’re facing.

  • Breathing difficulty

If your betta is experiencing difficulty breathing, it might be due to a bacterial infection like furunculosis.

In such conditions, the fish may start gasping for air on the water’s surface.

You may notice respiratory distress, making them breathe rapidly and shallowly.

  • Fin clamping

Betta fish may respond to the discomfort and pain they feel due to bacterial infection by clamping their fins tightly to the body.

So, if you notice clamped fins in your betta, it might indicate furunculosis.

  • Bloody discharge from body areas

When furunculosis is about to reach its final stage, it may cause bloody discharge from different body areas of the fish, especially the nostrils.

So, you should act immediately for the well-being of your betta fish if you notice signs of blood discharge on their skin.

  • Popeye

Popeye is a condition in which betta fish have swollen or bulging eyes. It may appear as a symptom of furunculosis when the infection occurs in the skin area around the eyes.

However, popeye is one of the less common symptoms of furunculosis.

Besides these noticeable symptoms, betta fish may develop some microscopic pathological manifestations, such as:

  • Enlarged spleen
  • Focal necrosis of the liver
  • Congested intestine
  • Growth of bacterial colonies in/around body tissues

Betta fish affected with furunculosis may suffer from digestive issues due to the filling of their stomach with mucus produced by the body as a defensive response to the infection.

Such issues can be diagnosed by examining the faeces of the affected fish.

Treating furunculosis in bettas

As furunculosis is a bacterial infection, antibacterial medications are considered the primary treatment for it.

However, there are many other treatments applied by aquarists to save the lives of their bettas, such as aquarium salt treatment, water changes,, etc.

Here are some common and potentially effective treatments to cure furunculosis in betta fish:

  • Antibiotic medication

Antibiotics, such as erythromycin or tetracycline, can treat betta fish suffering from furunculosis.

You can administer these medications by mixing them in your fish’s food (if they’re still eating) or aquarium water.

  • Aquarium salt treatment

Aquarium salt is believed to be effective in boosting the immune system of betta fish, increasing their defensive capability against various health conditions, especially bacterial infections.

You can treat furunculosis in your betta fish by giving them an aquarium salt bath. Follow these steps for salt bathing your fish.

It’s essential to remember that salt concentration in the water of the quarantine tank should not exceed the recommended dosage because too much aquarium salt can harm fish’s health.

  • Regular water changes

Performing regular water changes in your aquarium can help you prevent harmful bacteria from blooming in the aquarium.

When the aquarium water is changed regularly, it reduces the risk of reinfection as most of the harmful bacteria are eliminated from the water.

It’s recommended to change at least 25% of the water every week. However, you can do it more frequently if required.

How to prevent furunculosis in betta fish

Furunculosis is a potentially lethal condition that you should take seriously to keep your bettas healthy and happy.

Its treatment is relatively more complex than other health conditions, making it important to prevent this condition from happening in the first place.

Here are some useful tips that can help you prevent furunculosis in your betta fish:

  • Well-maintained, clean water: Bacteria that cause furunculosis thrive in dirty and poorly maintained water. Therefore, the first thing you can do to prevent furunculosis is to keep the water in your aquarium well-maintained and clean. Perform regular water changes by consulting with your veterinarian.
  • Quarantine new fish: Quarantine your newly adopted fish for at least two weeks before introducing them into your main aquarium. It will allow you to monitor their health condition and help you ensure that they’re not a carrier of any disease or bacterial infection.
  • Avoid overcrowding: Overcrowding is a stress factor for betta fish because it decreases their free space in the aquarium. A stressed betta has a compromised immune system, increasing the risk of bacterial infection. Therefore, you must avoid overcrowding in the fish tank.
  • Adequate nutrition: If you want your bettas to effectively fight off bacterial infections, they must be appropriately nourished, which is only possible if you provide them with a balanced, high-quality diet. Your betta’s diet should be rich in protein sources, meaty and non-meaty contents, and essential nutrients.


Although furunculosis is a life-threatening condition, you can keep your bettas healthy and safe from this condition by following the above-mentioned tips.

It’s also important to have basic knowledge about the causes and symptoms of furunculosis, so you can act immediately if you suspect your bettas are suffering from this serious condition.

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