Betta Fish Tumor – Symptoms, Causes, And Treatment

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You love your betta buddy, and if he suddenly develops a white bump on his side, does that mean he has cancer? 

Tumors on fish are generally symptomatic of an underlying health condition. However, in many cases, a betta fish tumor can be treated successfully, provided you spot the problem early and take appropriate action.

So what causes tumors to form on betta fish? And how do you treat a lump on your betta?

Read this guide to learn how to identify and treat betta fish tumors to save your fishy friend.

How Likely Is It That A Tumor Will Grow On Your Betta Fish?

Cancerous tumors on betta fish are actually quite rare. In fact, it’s far more likely that an injury or a disease will cause a lump or bump on your pet. So, if you spot something that looks like a tumor, don’t panic, that might be symptomatic of something else entirely. 

Basically, anything that manifests as a bump or swelling can be mistaken for a tumor on your betta. So, that includes bloating, Swim Bladder disease, or even constipation. Sometimes, a bacterial infection can morph into an ulcer or abscess that might look like a tumor, and small cottony growths caused by fungus might appear as bumps to the naked eye.

Finally, a fish tank that has poor water conditions can produce tumor-like symptoms, and a nip from an aggressive tank mate might also cause a lump on your betta’s body.

How To Know If Your Betta Fish Has A Tumor?  

So, as we established above, there are a few things that can be taken for tumors on betta fish.

Simple spots and bumps could turn out to be tumors. Tumors can appear under the fish’s skin, on the outside of the body on the scales, and even out of sight inside your betta’s body on his vital organs and other tissues.

If there’s no obvious lump on the outside of your pet’s body, other symptoms can indicate that he has an internal tumor of some kind.

Betta Tumor Or Not?

There are several other conditions that are commonly mistaken for tumors on betta fish. 


If you see a white growth on your betta fish, that’s most likely an abscess that might be caused by a bacterial infection rather than a cancerous tumor.

Bacterial infections usually emanate from exterior injuries, such as bites from other fish, bumping into tank decorations, or injuries picked up during water changes and tank cleaning. Injuries can also occur through the fish deliberately rubbing against objects in the tank.

Minor wounds generally heal themselves over time without treatment. However, dirty water can allow bacteria to get into the wound, triggering infection and setting up an abscess. Abscesses will simply get bigger and bigger until they rupture, leaving the betta with a large hole in its side or ragged fins.

Fungal Infections

Fungal infections often cause discoloration of the fish’s scales. Unlike abscesses, fungal infections usually appear like fluffy cotton. 


Ulcers commonly appear as white lumps on the fish’s body. Often, these lumps gradually develop into open, bleeding wounds.

These wounds can cause permanent disfigurement and scale loss, as well as providing an entry point for bacteria or fungus.

Generally, most bacterial infections, fungus, and ulcers can be treated successfully with antibiotics or another appropriate disease-specific over-the-counter medication.


Columnaris is a serious condition that often causes bumps on and around the fish’s head scales that can look like tumors at first glance.


Sometimes, parasite activity can cause tissue damage that can manifest as lumps and bumps on the betta’s body or head.

Swim Bladder Disease

Sometimes, Swim Bladder Disease can make it appear that the betta fish has a large lump on one side of its belly. 


In a similar way to Swim Bladder Disease, constipation can cause swellings to appear around the fish’s belly, and the betta will be unable to swim upright or on an even keel.

Usually, constipation can be treated by withdrawing food from your betta for a couple of days and then feeding him live or frozen meaty foods. 

Gill Hyperplasia

Gill hyperplasia can cause damage to the structure of the gills, causing them to shrink and become malformed as they slowly heal. That can produce a swelling that appears to be a tumor at first glance. In many cases, that disfigurement is permanent.

Where Can Tumors Be Located?

As discussed above, lumps and bumps that appear to be tumors can be located pretty much anywhere on your fish’s body, including:

  • On your betta’s head
  • On the sides of the body
  • On the fins and tail
  • On the fish’s belly
  • On the gills

Generally, tumors that develop inside the fish are cancerous and untreatable. However, the lumps and bumps that you see elsewhere on your pet are usually attributable to other less sinister causes and are not malignant.

What Causes Tumors In Betta Fish?

Just like people, some fish are simply predisposed to developing cancerous tumors. So, essentially, there’s nothing you can do about it.

That being said, there are few other things that might cause tumors to develop. For example, allowing carcinogenic substances to get into your tank water, offering your fish a poor diet, and failing to maintain the environment correctly. Dirty water can weaken the fish’s immune system, leaving him vulnerable to viral infections that could ultimately cause tumors.

How To Treat A Tumor In A Betta Fish

Unfortunately, if your betta fish develops a malignant tumor, there’s not really much you can do about it. 

If your pet develops a tumor, you must think about his quality of life. Provided that the betta is swimming around and is able to feed, it’s probably worth letting him live for a while longer. That being said, if your fish appears distressed and or disinterested in food, it’s time to euthanize him.

There are cases of people successfully having their betta fish operated on to remove tumors, but that’s the exception rather than the rule.

When Euthanizing Your Betta Is The Kindest Thing To Do

Sometimes, the only kind course of action to take when your betta develops a tumor that can’t be cured is to humanely euthanize him.

image of black goldfish died in freshwater aquarium tank.

A quick Google search will bring up numerous articles about the best way to euthanize your fish. However, from personal experience, I recommend using a product such as Fin-Quel or something similar that you can obtain from your vet or from good fish stores. These products are actually intended for use as sedatives, but an overdose will quickly and peacefully send your betta on his way to fishy heaven. 

However, before you do the final deed, always seek expert advice from your veterinary surgeon.

Treating Abscesses

If you’re confident that your betta’s lump is actually an abscess rather than a tumor, you can take positive steps to treat it.

Remove your pet from the main aquarium and put him into a quarantine tank on his own. The quarantine tank should have pristine water so that when the abscess ruptures, no bacteria will get into the wound and set up an infection.

Set up the quarantine tank with plenty of silk plants and a cave or smooth-sided plastic pot in which your pet can take shelter. Bettas can get very stressed when they’re sick and feeling vulnerable, so you need to make sure that your fish has a safe hideout. Silk plants are a better choice than real ones, as there’s a much lower risk that bacteria will get into the quarantine tank.

Treat the water with an appropriate over-the-counter aquarium medication that you can get from your local fish store. Follow the manufacturer’s dosage instructions carefully and be sure to complete the whole course of treatment.

How To Treat An Ulcer

If your betta develops an ulcer, that’s treatable! 

Immediately remove your pet from his main tank and put him into a quarantine setup as described above. Ulcers can develop into nasty open sores, providing an entry point for bacteria and fungal infections. While your fish is in the quarantine tank, keep everything scrupulously clean and carry our water changes several times a week.

Following the first water change, treat the tank with aquarium salt at a rate of ¼ ounce per gallon of water. That will accelerate healing, as well as acting as a mild disinfectant. After each water change, you’ll need to add 30% of the quantity of aquarium salt that you added before.

You also need to use an antibacterial medication to protect your fish from complications and secondary infection.  

Once the ulcer has completely cleared up, you can return him to his main tank. 

Can You Prevent Tumors In Betta Fish?

Whether your betta gets a tumor or not is largely determined by genetics. That being said, there are a few things that you can do to protect your fish from developing a tumor.

Buy Your Betta From A Reputable Breeder

When you buy your betta fish, you most likely do so from a local fish store or online breeder. However, I recommend that you always source your betta fish from a reputable breeder. 

Often, breeders that produce large numbers of fish for the trade inbreed their stock because it’s cheaper than researching genetics and breeding the fish properly. The result of that inbreeding and lack of genetic screening has led to a whole host of problems, one of which being an increased chance of tumors.

Keep Your Betta’s Tank Clean!

You should always keep your betta’s tank clean. 

betta fish, siamese fighting fish in aquarium

Dirty water and a poorly maintained filtration system is pretty much guaranteed to make your betta sick. Bettas are hardy fish but they are quite sensitive to water conditions and temperature. If you don’t keep up with your tank cleaning, weekly water changes, and filter maintenance, your betta’s immune system will be compromised, leaving him vulnerable to attack by bacteria and parasites.

To reduce the risk of gill hyperplasia and possible gill tumors, ensure that ammonia and nitrite levels in your tank are at zero.

Also, bettas are labyrinth breathers. To avoid damage to the labyrinth organ, the ambient temperature in the room where your betta’s tank is must be the same or very close to that of the tank water.

Feed Your Betta A High-Quality, Balanced Diet

Bettas can be picky eaters at the best of times, and feeding a poor-quality diet is sure to cause health issues, including tumors.

In the wild environment, your betta buddy enjoys a diet that’s primarily made up of meat with a small quantity of plant matter. Your fish needs that balance in his diet to keep him healthy and enable him to fight off stress-related diseases. It’s also thought that eating a high-quality, correctly balanced diet can help to reduce the risk of cancer in people, so the same is likely for your betta. 

Treat Other Diseases Promptly

If your betta fish gets sick, be sure to treat the problem right away. The more time that elapses, the sicker your fish will get, and he could even die.

Sometimes, the stress of being poorly can leave your betta fish exposed to the risk of developing a tumor. So, if your betta develops any form of disease, be sure to treat him immediately.

Keep Your Aquarium Free From Carcinogens

There are some chemical substances that are known to cause cancer in pet fish. So, before you add anything to your fish tank, be sure to check that the substance is safe and doesn’t contain anything carcinogenic.

In Conclusion

I hope you found our guide to tumors in betta fish helpful. If you did, please remember to share the article with other betta enthusiasts.

Thankfully, cancerous tumors in betta fish are quite rare. Sadly, if your fishy friend does get a malignant tumor, it’s unlikely that you will be able to save him. However, your betta might develop lumps and bumps that are caused by other diseases and accidental trauma. That,, many of which are totally curable and preventable.

Did your betta fish get a tumor? How did you treat it, and how is your betta buddy doing now?

Alison Page has been an avid fish keeper for over 35 years and has owned many different species of freshwater tropical fish including bettas. Currently Alison has two large freshwater tanks. The first tank has two huge fancy goldfish who are almost ten years old and still looking as good as ever. In the other, she has a happy community of tiger barbs, green tiger barbs, corydoras catfish, platys, and mollies.

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