Fish tumor or lump? What lumps and bumps on fish mean

A fish tumor is a lump caused by cells growing out of control, but many lumps on fish are not tumours at all. They can also be a virus (lymphocystis), a parasite, a cyst or an abscess, and some of those are treatable or clear up by themselves.

Most single, slow-growing lumps are not an emergency. Watch the lump, keep the water clean, and judge the fish by how it eats, swims and breathes. This guide helps you work out what the lump might be and when a vet can help.

How to identify it

Take a clear photo of the lump today, with something for scale, so you can compare it week by week. Then look at its shape, colour and speed of growth.

What you see	Likely cause
Grainy, white or cream "cauliflower" lumps on fins or skin, growing over days, in cichlids, gouramis or glassfish	Lymphocystis (a virus). Usually clears in weeks.
A firm, smooth lump that grows slowly over months	A tumour, such as a fibroma (a benign lump of fibrous tissue), especially in goldfish
A soft, shiny or silvery lump on a betta	Possibly an iridophoroma, a tumour of the shiny pigment cells
Black, spreading patches or lumps on platies, swordtails or their crosses	Possibly melanoma, which runs in some bred lines
A red, swollen, soft lump that may burst and leave an ulcer	An abscess or bacterial infection
Small raised black, white or yellow dots or bumps under the skin	Parasite cysts ("grubs"), common in wild-caught or pond fish
A worm or thread sticking out of the lump	Anchor worms
A lump on the lip of an angelfish	A lip fibroma, linked to a virus
Swelling under the jaw or throat	Possibly a thyroid swelling (goitre), linked in some fish to high nitrate or low iodine
A swollen belly on one side, slowly growing, in an older fish	An internal tumour, often of the egg or sperm organs, or egg binding. See bloated fish.
Swollen belly with scales sticking out like a pine cone	Dropsy. More urgent.

Only a vet can confirm a tumour, usually by taking a small sample (biopsy) or by examining the fish after death.

Goldfish and older fish

Tumours are more common in older fish and in some species and lines. Goldfish are known for fibromas and related skin lumps, and some of these are thought to be inherited. Koi are known for internal tumours of the egg or sperm organs, which show as a swollen or lopsided belly and gradual weight loss. Long-lived fish are more likely to develop lumps as they age, in the same way older dogs and people do.

When it's a problem

Usually fine to watch: A small lump that grows slowly, on a fish that eats, swims and breathes normally.
Act this week: A lump that is growing quickly, is red or bleeding, or is on the mouth, eye or gill cover.
Serious: The lump stops the fish eating, swimming upright or closing its gill covers, or the fish is losing weight.
Emergency: Scales sticking out, gasping, lying on the bottom, or an open, rotting wound. See dropsy and gasping.

What to do right now

Photograph the lump with a coin or ruler beside the tank for scale.
Test the water for ammonia, nitrite, nitrate and pH, and fix anything that is off.
Check the rest of the fish. Several fish with lumps points to a virus or parasite, not cancer.
Watch the fish eat. Note whether it can reach and swallow food easily.
Don't squeeze, cut or lance the lump. That causes wounds that easily become infected.
Move it to a hospital tank if other fish are pecking at the lump or bullying it.

What to do next

Watch and record

Take a new photo every week. Lymphocystis usually grows, then shrinks and falls away within a few weeks. Tumours tend to keep growing slowly. Abscesses often change quickly, swelling and then bursting.

Treat what can be treated

Lymphocystis: no medicine needed. Clean water and less stress. See our lymphocystis guide.
Abscess or infection: keep the water very clean. If it opens into an ulcer, treat with an antibacterial product in a hospital tank and remove carbon from the filter while medicating. Some infections need antibiotics, which require a vet prescription in many countries.
Parasite cysts: many do not spread in a home aquarium because the parasite needs a snail and a bird to complete its life. They often cause little harm.
Anchor worms: see our anchor worm guide.

Be careful with medicines around shrimp, snails, plants, loaches, and plecos and other scaleless fish. There is no medicine you can add to the water that shrinks a true tumour.

What a vet can do

Fish vets can do more than most people expect:

Biopsy: a small piece of the lump is taken under anaesthetic and examined to find out what it is.
Surgery: many skin lumps and some internal tumours can be removed under anaesthetic, especially in larger fish such as goldfish and koi. Removing an angelfish lip fibroma can let the fish eat normally again.
Imaging: ultrasound or X-ray can show whether a swollen belly is a tumour, eggs or fluid.
Pain relief and advice on quality of life.

Lumps often come back after removal, and surgery has risks. A vet can help you weigh them for your fish.

Quality of life

Many fish live comfortably for months or years with a slow-growing lump. Ask yourself:

Can it eat without struggling?
Can it swim upright and rest normally?
Is its breathing calm?
Is it holding its weight?

When the answer to these becomes "no", it is kinder to consider humane euthanasia. See our dropsy guide for a gentle method, or ask a vet.

Products that help

Liquid water test kit: rules out poor water as a cause of infections and stress.
Hospital tank with sponge filter and heater: a calm place for a fish with a lump that is being pecked.
Antibacterial treatment: only for open, infected sores or ulcers, used in a hospital tank.
Soft, sinking foods: help a fish with a lump near the mouth eat more easily.
Camera or phone with a ruler for scale: makes tracking growth easy.

How to prevent it

You cannot prevent every tumour, especially inherited ones and those in older fish. You can lower the risk of the other lumps:

Quarantine new fish for 4–6 weeks. See our quarantine guide.
Keep water clean and nitrate low. See nitrates.
Avoid rough handling and aggressive tankmates that cause wounds.
Feed a varied, good-quality diet.
Buy from breeders who do not breed from fish with tumours.

Why it happened

True tumours come from a mix of genes, age, viruses and environment. Some lines of goldfish, platies and swordtails are known to carry a higher risk. Other lumps come from infections and parasites that took hold after stress, injury or poor water.

When to get help

See an aquatic vet if a lump grows quickly, bleeds, affects the mouth, eyes or gills, or stops the fish eating, or if you have a large, long-lived fish like a goldfish or koi that may be a candidate for surgery. Bring your weekly photos.

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