betta fish tumor

Betta Fish Tumor: Causes, Cures, and More

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Most fish hobbyists have to cope with sick fish from time-to-time. That can be devastating, especially if the affected fish is your prized betta. The good news is that most common fish diseases can be cured. But what about betta fish tumors?

In this guide, we discuss how to identify a betta fish tumor, what treatment could save your fish, and what you can do to prevent more tumors affecting your pet.

How to recognize a betta fish tumor

If you notice lumps or bumps on or under your betta fish’s skin, that’s most likely a tumor. Tumors can be tiny and barely noticeable, or they can be huge. Tumors covering large areas of the fish’s body can cause problems with buoyancy and affect the fish’s ability to swim. 

However, some tumors are internal, and those can’t be seen by the naked eye. Internal tumors usually impact on the fish’s ability to eat and swim. That causes a rapid decline in the fish’s health. Unfortunately, with these, there’s usually nothing you can do to save your pet. 

In cases like that, the only course of action that you can take is to euthanize your betta humanely. 

What causes tumors?

Some bettas are predisposed to developing cancerous tumors, just like some people do. That’s the most common cause of this condition.

However, there are a few causes of tumors in a previously healthy betta. These include a poor diet, unsanitary water, and viral infections. The last one may be introduced into the tank when new specimens are added to the environment. 

Can you treat betta fish tumors?

External cancerous tumors can be removed from betta fish. But this highly risky surgery should only be undertaken by a qualified, experienced veterinary surgeon who is a specialist in treating tropical fish. And there’s no guarantee that the tumor will not recur in the future

You can see a video of veterinary tumor removal from a betta fish by clicking this link.

When is a lump not a tumor?

However, there is some good news. Chances are, what you’re seeing is not a tumor. Cancerous tumors in betta fish are quite rare. If you see a lump or bump on your betta fish, it’s more likely to be an ulcer or an abscess, both of which can look like tumors at first glance.

Other causes of lumps that may be mistaken for tumors include fluid retention, swim bladder disease, and constipation.

How to recognize a betta fish abscess

If your betta fish develops a white lump on his body, it’s most likely an abscess, not a cancerous tumor.

Abscesses are caused by bacterial infections. It could be that your betta has sustained an injury by catching himself on something sharp in the tank, or the damage may have been caused by a nip from another fish. Most times, injuries like that clear up on their own. But, if the tank water is not kept in pristine condition, bacteria can enter the wound site and set up an infection. 

As the bacteria destroy the tissues surrounding their entry point, the decomposing matter forms pus. The pus collects beneath the fish’s skin, forming a lump or abscess.

If left untreated, an abscess will continue to grow until the fish’s skin is no longer able to contain it, at which point the abscess will burst. Once the abscess has ruptured, the fish will be left with a large, open sore on his body, leaving him vulnerable to further attack by bacteria, and so the whole cycle begins again.

How to treat a betta fish abscess

As soon as you notice what could be an abscess forming on your fish, remove him from the main tank and place him alone in a quarantine tank. 

A quarantine tank is simply a tank that has the fish’s preferred water quality, a good filtration system, heating, lighting, and a hiding place. Keep the tank scrupulously clean, and carry out frequent water changes, especially once the abscess has burst.

Providing a cave or something similar in the quarantine tank is very important. A sick fish will become stressed if he has nowhere to hide while he’s feeling vulnerable. A smooth-sided, plastic plant pot set on its side and partially buried in the substrate makes a safe, affordable hideout for your betta.

Plants make a great place for fish to hide and recuperate. However, it’s better to use silk plants, rather than fresh ones, in your quarantine tank. Fresh plants could bring unwanted bacteria or parasites into the environment that could seriously harm an already sick fish.

If the abscess is not too big, your betta has a good chance of surviving. Treat the tank water with a suitable aquarium antibacterial product that you’ll find in good fish stores or online. Take care to follow the manufacturer’s instructions and be sure to use the correct recommended dose.

How to recognize a betta fish ulcer

Sometimes, betta fish develop ulcers, which manifest themselves as lumps on the skin. Ulcers are usually easily identifiable due to the sore-looking, reddened areas around the edges of the lump.

Fish affected by ulcers often appear lethargic and emaciated to the point that they stop eating completely.

Fish ulcers are usually caused by bacterial infections. Ordinarily, many bacteria are already present in a healthy aquarium and cause no harm to healthy fish. However, once the fish become stressed by adverse environmental conditions or poor water quality, they become vulnerable to attack by the bacteria, which may cause ulcers.

How to treat a betta fish ulcer

As soon as you suspect that your betta has developed an ulcer, remove him from the main aquarium and place him into your quarantine tank.

Even if the ulcer clears up, fungal infections frequently follow. So, it’s vital that you carry out frequent water changes for two to three weeks following the successful treatment of the ulcers.

After the first water change, add salt to the tank water at one-quarter of an ounce per gallon. That should encourage wound healing by reducing the osmotic effect of water entering the ulcer and also having a mild disinfectant action.

Maintain the salt content of the water after each water change by adding 30% of the quantity of salt you originally added. You can monitor the salt levels in the water by using a hydrometer. You should also add an aquarium antibacterial product to the water. 

Wait until the ulcer has completely healed before returning your betta to the main display tank.

Location, location, location!

Tumors can appear anywhere on your betta fish’s body. However, as we’ve already mentioned, these lumps might not be caused by tumors. In fact, lumps on certain parts of your betta’s body are most likely not tumors at all.

Here’s a brief guide to help you work out what causes lumps to appear on particular areas of your betta’s body: 

Lumps on betta’s head

One very common place to find lumps is on your betta’s head.

Although head tumors can occur in bettas, lumps in this location can also be caused by a bacterial infection. Columnaris is one common bacterial infection that causes lump-like lesions to form around the fish’s gills and mouth. 

Lumps on betta’s side

Lumps on your betta fish’s side can be caused by a variety of things, most of which are treatable. Of course, a lump could be a cancerous tumor, but it’s much more likely to be one of the following conditions:

Swim bladder disease

Swim bladder disease can give your betta’s side a swollen, lumpy appearance.

The swim bladder is an internal sac filled with gas that helps the fish to move up and down in the water, in the same way as a diver might use a buoyancy aid. 

Fish that are affected by the disease either sink to the bottom of the tank or float involuntarily at the surface. Sometimes, an affected fish will be unable to keep itself stable and will adopt a lopsided position while swimming. Also, the fish’s abdomen will appear swollen, and he will be more lethargic than usual.

Overfeeding or constipation usually causes these swim bladder issues. You can often treat swim bladder disease successfully by withdrawing food from the fish for a few days to allow his digestive system to process any food that’s still inside. 

Swim bladder disease is not contagious, so there’s no need to isolate your betta. There are also some proprietary treatments that you can buy over the counter for the treatment of swim bladder disease that should sort out the problem pretty quickly.


Dropsy is a serious bacterial infection of the betta’s kidneys, causing fluid to accumulate, which eventually leads to renal failure. The condition is likely caused by unsanitary aquarium conditions.

Betta fish with dropsy appear bloated, have curvature of the spine, and, in the disease’s final stages, “pinecone” or protruding scales. Although you can try to treat dropsy with antibacterial water treatments, the prognosis is generally poor.

Bacterial infections

Fish that have injured themselves or been nipped can develop bacterial infections, which often appear as lumps on the fish’s sides. Those lumps can turn into abscesses, as described above.

If you’re concerned that your tank is infected, you can try treating the water with an over-the-counter antibacterial product, which should clear things up relatively quickly. In the meantime, quarantine your fish in a separate tank.

Lumps on betta’s stomach

The final most common place to find tumors on your betta fish is on his stomach. Although such lumps can be tumors, there are other likely culprits too. Dropsy, swim bladder disease, or bacterial infection can all manifest on the stomach in addition to the sides. Also, constipation may cause swelling or lumps in this area.


Constipation can cause bettas to develop lumps on their stomachs. It is a very common condition that affects bettas, but, luckily, it’s extremely easy to treat. Constipated fish usually stop eating, become lethargic, don’t pass any feces, and can even develop swim bladder disease.

You can cure constipation by withdrawing food from your betta for a couple of days and then offering live or frozen food, such as bloodworms or mosquito larvae, rather than dry pellets or flakes. That’s usually enough to kickstart your fish’s system again and relieve the blockage.

By including one “fasting” day each week in your betta’s feeding regime, you can help to prevent constipation from affecting your fishy friend.

Tumors in betta’s gills

If you notice a lump or tumor in your betta fish’s gills, it’s often a sign of gill hyperplasia.

Gill hyperplasia usually occurs following damage to the gills. That could be caused by a parasitical or bacterial infection, physical injury, or by toxins, such as ammonia, nitrates, and nitrites. Rather than healing properly, the new gill tissue grows over the damaged area, forming a lump that can resemble a tumor. Over time, the lump grows bigger as more new skin forms.

In severe cases of hyperplasia, the lumps are permanent. However, in minor cases, the lumps disappear, and the gills return to normal.

How to prevent tumors in bettas

In fish that are not predisposed to genetically inherited tumors, there is much you can do to prevent the likelihood of your betta being affected.

  1. When buying a betta, always source your fish from a reputable breeder. Some fish stores inbreed their stock because that’s cheaper than importing or rearing bettas properly. Inbred fish are highly likely to come with a whole gamut of problems, including an increased risk of developing cancerous tumors.
  2. Keep your tank water in pristine condition by using and maintaining an efficient filtration system. Be sure to change 25% of the water every week to prevent bacteria from building up, which could potentially put your fish at risk of developing health problems.
  3. Feed your betta only good quality food. Low-grade foods usually do not contain the high levels of protein and nutrients that your betta needs to stay in good health. Check out our guide to feeding your betta fish for the full low-down on the dos and don’ts of betta nutrition.
  4. If any of the occupants of your betta setup become sick, be sure to remove the affected fish right away and put them in a quarantine tank. That way, you can treat the sick fish and hopefully prevent a contagious condition from spreading.

Final thoughts

Although cancerous tumors appear in bettas, there are many other causes of lumps and bumps that you might notice on your fish’s body, many of which respond very well to correct and timely treatment.

Always buy your betta from a reputable fish shop. Ideally, go with one a friend or other reputable, knowledgeable source recommends to you. Feed your betta only the best quality food, keep your fish’s tank clean, and always quarantine new fish before adding them to your betta’s habitat.

Finally, always check the tank’s occupants every day for signs of injury or disease. Spotting a problem early can mean the difference between life and death for your precious betta.

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