My Betta Fish Has a Lump or White Bump on Its Side

Name: My Betta Fish Has a Lump or White Bump on Its Side
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-10-02

If your betta fish has a lump on its side, the cause can range from a harmless color change to a tumor, an abscess, or a parasite. Some lumps need no treatment, while others need a veterinarian's help or humane euthanasia. The first step is to look closely at the lump and your fish's overall behavior.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Answer

A small, smooth, flat spot that matches your betta's color is often normal pigment, especially in marble or koi bettas.
A raised, firm, growing mass is more likely a tumor, and tumors in fish are well documented [1].
A soft, red, or pus-filled swelling may be an abscess or a bacterial infection that needs treatment.
Any lump plus gasping, not eating, or lying on the bottom is a reason to contact a veterinarian who treats fish.
Do not poke, squeeze, or cut the lump at home. You can cause bleeding, infection, or death.

What the Lump Looks Like and What Is Normal

Bettas change color for many reasons. A marble betta can develop new patches of white, blue, or black over weeks. A flat white patch that does not rise above the skin and does not grow is usually pigment, not disease.

A true lump is raised. It may sit on the skin, under the skin, or inside the body wall. It can be white, pink, red, black, or the same color as the fish. It may be smooth, bumpy, firm, or soft.

Some lumps are harmless for months. Others grow fast and change the fish's shape. The speed of growth matters more than the size at first glance.

Watch for other signs at the same time. A betta that still eats, swims, and flares is in better shape than one that hides, gasps at the surface, or lies on its side. If your betta has stopped eating, see Betta Fish Not Eating for what to check first.

Why Is My Betta Fish Turning White and Laying on Its Side?

This combination of signs is more serious than a lump alone. Turning white can mean pigment loss, but it can also mean poor blood flow, infection, or organ failure. Lying on the side means the fish is too weak to hold itself upright.

Check your water first. Ammonia, nitrite, and temperature problems cause sudden color loss and listlessness. Test the water and do a partial water change with a conditioner.

If the water is normal and the fish is still pale and lying down, the fish is likely very sick. This is a same-day call to a veterinarian who sees fish. Do not wait a week to see if it improves.

Main Causes of Lumps and Bumps on Bettas

Cause	Typical clues	How urgent
Pigment change (iridophore or melanophore patches)	Flat, smooth, matches scale pattern, grows slowly or not at all	Not urgent, monitor
Benign tumor (fibroma, chondroma)	Firm, round, slow growing, fish acts normal	Routine visit if it grows
Malignant tumor (irido-melanophoroma, chromatophoroma)	Fast growing, irregular, may invade tissue, fish declines	Urgent, see a vet
Abscess or bacterial infection	Soft, red, warm-looking, may drain, fish may be lethargic	Urgent, needs treatment
Parasites (external or internal)	Bumps, flashing, rubbing, weight loss, visible worms	Urgent if fish is declining
Mycobacterial infection	Multiple nodules, wasting, chronic decline	Urgent, hard to treat

Pigment Changes

Bettas have pigment cells called iridophores and melanophores. These cells can cluster and form raised spots that look like tumors but are not. Some of these pigment cell growths, called chromatophoromas, have been reported in bettas since 2015 [2].

A flat or slightly raised patch that stays the same size for months is often just pigment. Take a photo every week with a ruler next to the fish so you can compare.

Tumors

Tumors are common in fish, and some are linked to genetics [1]. In bettas, pigment cell tumors are the most reported type. A recent case described a malignant mixed irido-melanophoroma in a 10-month-old male betta with multiple masses on the flank [3]. That tumor grew fast and invaded tissue.

Not every tumor is cancer. Some are benign, like the fibroma seen in other fish species [1]. Benign tumors grow slowly and do not spread. Malignant tumors grow fast, change shape, and can make the fish thin or weak.

There is no way to tell a benign tumor from a malignant one just by looking. A veterinarian can take a sample and send it to a lab.

Abscesses and Bacterial Infections

An abscess is a pocket of pus under the skin. In fish, abscesses often look like soft, raised, red or white bumps. They can be caused by bacteria that enter through a small wound or a parasite bite.

Mycobacteria are a group of bacteria that can cause multiple nodules in bettas. One study found Mycobacterium chelonae in a betta with a tumor, which shows that infection and tumors can happen together [3]. Another report linked multiple bacterial infections, including four Mycobacterium species, to a skin nodule syndrome in bettas [2].

Abscesses need veterinary care. A veterinarian may drain the abscess, prescribe antibiotics, or both. Do not try to drain it yourself.

Parasites

Parasites are a major cause of lumps in fish [4]. Some parasites form cysts under the skin that look like white bumps. Others live inside the body and cause swelling.

External parasites like Ich cause white spots, but those spots are small and uniform, not single lumps. If you see many tiny white spots, that is a different problem. See Betta Fish White Spot Disease (Ich): Treatment and Prevention for that pattern.

Internal parasites can cause weight loss, a sunken belly, or a lump in the body cavity. A veterinarian can examine a stool sample or a skin scrape to find them.

Other Causes

Not every lump is a tumor or infection. Some are fluid-filled cysts, scar tissue, or a reaction to an injury. A fish that scraped itself on a sharp decoration can develop a swollen area that looks like a lump.

Egg binding in females can cause a swollen belly, but that is usually in the belly, not the side. If your betta has a swollen belly, see Betta Fish Bloated Belly for that specific sign.

Red Flags That Make It an Emergency

Call a veterinarian who treats fish right away if you see any of these:

The lump grew noticeably in a few days.
The lump is bleeding, open, or draining.
The fish is gasping at the surface or lying on its side.
The fish has stopped eating for more than two days.
The fish is pale, thin, or has a sunken belly.
The lump is blocking the fish from swimming or eating.
Multiple lumps appeared at once.

These signs mean the fish is in distress. Waiting can make treatment harder or impossible.

Safe Things You Can Do at Home While Waiting

You cannot treat a tumor at home. You can make your betta more comfortable and avoid making things worse.

Check and Fix the Water

Test ammonia, nitrite, nitrate, pH, and temperature. Ammonia and nitrite should be zero. Nitrate should be under 20 ppm. Temperature should be 78 to 80°F for bettas.

Do a 25 to 50 percent water change with a conditioner if any level is off. Match the new water temperature to the tank.

Keep the Tank Calm

Lower the light if the fish is hiding. Reduce traffic around the tank. Do not add new fish.

Do Not Poke or Squeeze the Lump

You can cause bleeding, infection, or internal damage. You can also spread bacteria into the water. Leave the lump alone.

Keep Feeding Simple

Offer small amounts of food. If the fish is not eating, remove uneaten food so it does not foul the water. Do not force feed.

Take Photos and Notes

Take a clear photo of the lump every two to three days. Note the date, size, color, and any behavior changes. This helps the veterinarian see how fast it is changing.

Isolate if Needed

If other fish are in the tank, move the betta to a hospital tank. This protects both the betta and the other fish. Use a filter and heater in the hospital tank.

What the Veterinarian Will Check and Test

A fish veterinarian will start with a physical exam. They will look at the lump, check the fish's body condition, and watch how it swims.

They may ask about your water parameters, diet, tank mates, and how long the lump has been there. Bring your photos and notes.

Common Tests

Skin scrape or fin clip to check for parasites [4]
Impression smear of the lump to look at cells
Biopsy or fine needle aspirate to check for tumor cells
Bacterial culture if infection is suspected
Water quality testing
Ultrasound or radiographs in some cases

A biopsy is the only way to know for sure if a lump is a tumor and whether it is benign or malignant. A veterinarian may send the sample to a lab.

Treatment Options

Treatment depends on the cause. Options include:

Monitoring if the lump is small and the fish is healthy
Antibiotics for bacterial infections
Antiparasitic drugs for parasites
Surgery to remove a tumor, if the fish is strong enough
Debulking a mass that blocks eating, as described for a fibroma in an angelfish [1]
Humane euthanasia if the fish is suffering and treatment is not possible

A veterinarian calculates drug doses from the fish's weight and health. Never guess a dose on your own.

Prevention and Monitoring

You cannot prevent every tumor. Some are genetic [1]. You can lower the risk of infection and parasites.

Keep water clean. Test weekly and change 25 percent of the water weekly.
Quarantine new fish for two to four weeks before adding them to a tank.
Do not overcrowd the tank.
Feed a varied diet with quality betta pellets and frozen foods.
Remove sharp decorations that can cut your fish.
Watch your betta daily for changes in color, appetite, or swimming.

If your betta has a lump, measure it and photograph it every week. Write down the date. If it grows, changes color, or starts to bleed, call a veterinarian.

For a broader look at betta health problems, see Common Betta Fish Diseases and How to Treat Them.

Limitations and When to Contact a Veterinarian

No article can tell you what your betta's lump is. Only a veterinarian can examine the fish, run tests, and give you a diagnosis. Individual cases need a veterinarian.

Contact a veterinarian the same day if:

The lump appeared suddenly or grew fast.
The fish is not eating or is losing weight.
The fish is pale, gasping, or lying on its side.
The lump is bleeding or open.

Go to an emergency clinic now if:

The fish is gasping at the surface and cannot stay upright.
The lump is bleeding heavily.
The fish is unresponsive or floating on its side.

Schedule a routine visit if:

The lump is small, slow growing, and the fish acts normal.
You want to know if the lump is a tumor.
You want to discuss treatment options.

If treatment is not possible and the fish is suffering, talk to your veterinarian about humane euthanasia. This is a valid choice when a fish cannot eat, swim, or breathe without distress.

Frequently Asked Questions

Is a lump on my betta always cancer?

No. Many lumps are pigment changes, benign tumors, abscesses, or cysts. A biopsy is the only way to know for sure. Some tumors in fish are benign, like fibromas, while others are malignant [1].

Can I treat a betta tumor at home?

No. There is no home treatment that removes a tumor. You can keep the water clean and the fish comfortable, but a veterinarian must diagnose and treat the lump. Do not use human medications or antibiotics in the tank.

How fast do betta tumors grow?

It varies. Some grow slowly over months. A malignant irido-melanophoroma in one betta grew rapidly and invaded tissue [3]. Take photos every week so you can see the growth rate.

What does a white bump on a betta's side mean?

It can be a pigment spot, a parasite cyst, an abscess, or a tumor. A flat white patch is often pigment. A raised white bump that grows needs a veterinarian's exam. Multiple white bumps can mean a bacterial infection like mycobacteria [2].

Should I euthanize my betta with a lump?

Only if the fish is suffering and treatment is not possible. Signs of suffering include not eating, gasping, lying on the side, and being unable to swim. Talk to a veterinarian who treats fish before making this decision.

References

Neoplasia of Fish - Exotic and Laboratory Animals. Merck Veterinary Manual.
Histopathological description of iridophoromas resembling skin nodule syndrome in Siamese fighting fish Betta splendens. Diseases of aquatic organisms, 2022.
Malignant Mixed Irido-Melanophoroma in a Betta splendens with Concurrent Mycobacterium chelonae Isolation-A Case Report. Journal of fish diseases, 2025.
Parasitic Diseases of Fish - Exotic and Laboratory Animals. Merck Veterinary Manual.