Lymphocystis in fish: cauliflower growths, causes and care

Lymphocystis is a viral infection that causes white, cream or grey, cauliflower-like lumps on a fish's fins or skin. It looks alarming, but it is rarely fatal and usually clears up on its own within a few weeks once the fish is in clean water with less stress.

There is no medicine that cures it, and you should not try to cut or scrape the lumps off. Your job is to confirm it really is lymphocystis, make the fish's life as easy as possible, and stop it spreading to other fish.

How to identify it

Lymphocystis is caused by an iridovirus (a family of viruses that infect fish, frogs and insects). The virus makes individual skin cells swell to enormous size, so large that each one can be seen with the naked eye. Clusters of these giant cells form the lumps.

Look: Raised, rough, grainy lumps, like tiny cauliflower florets, cauliflower rice or a bunch of grapes. Usually cream, white or pale grey, sometimes pinkish.
Where: Most often on the edges of fins, then the body and around the mouth. Less often on the gills or eyes.
Size: From a few grains to a clump several millimetres across.
Behaviour: Most fish act normally, eat and swim as usual.

Which fish get it

More than 125 species can catch lymphocystis, mostly "advanced" bony fish. In aquariums it is most common in cichlids (including angelfish and rams), gouramis, glassfish, killifish and many marine fish such as clownfish. It does not infect catfish (including plecos and corydoras), cyprinids (goldfish, koi, barbs, danios) or salmon and trout. So if your pleco or goldfish has a white lump, it is something else.

Lymphocystis or something else?

What you see	Likely cause
Grainy, cauliflower-like lumps that grow slowly over days or weeks	Lymphocystis
Fluffy, cotton-wool tufts that wave in the water	Fungus or columnaris
Many small, even white dots like salt grains all over the body	Ich (white spot)
A smooth, firm lump under the skin that keeps growing for months	A tumour or cyst
A thread or worm sticking out of the skin	Anchor worms

Other things that can look similar include mucus tags, parasitic cysts under the skin ("grubs") and some skin cancers. A vet can confirm lymphocystis by looking at a small sample under a microscope, where the giant cells are easy to see.

When it's a problem

Mild (most cases): A few small lumps on the fins of a fish that eats and swims normally. Not urgent, but act this week to reduce stress.
Serious: Large clumps that weigh down fins, cover the mouth so the fish cannot eat, or cover the eyes.
Emergency: Lumps on the gills with fast breathing or gasping, or open sores where lumps have broken off and the area turns red or fuzzy (a secondary infection).

What to do right now

Test the water for ammonia, nitrite, nitrate and pH. Fix any problem you find. See ammonia and nitrates.
Do a 25–30% water change with dechlorinated, temperature-matched water.
Look for the stress behind it. Is the fish being chased, nipped or crowded? See aggression and stress.
Move the fish to a hospital tank if it is being bullied, or if you have other susceptible fish such as cichlids or gouramis. See our hospital tank guide.
Do not cut, scrape or pick at the lumps. That leaves an open wound that bacteria and fungus can invade, and can release virus into the water.
Take a clear photo so you can compare progress each week.

What to do next

Expect the lumps to grow for a while, then shrink and fall away. In tropical fish this usually takes a few weeks. Fish kept in cooler water may take about six weeks or longer.

Keep the water excellent. Weekly water changes of 25–50%, steady temperature, and no ammonia or nitrite.
Feed a varied, high-quality diet to support the fish's immune system.
Reduce stress: plenty of hiding places, peaceful tankmates and no overcrowding.
Watch for secondary infection. If a lump falls off and leaves a red, ragged or fuzzy sore, treat that problem, for example with a fin rot or fungus treatment, in the hospital tank. Remove carbon from the filter while you medicate.
Don't use antibiotics or antiviral "cures". They do not affect the virus. Medicines added to a main tank can harm shrimp, snails, plants, loaches, and plecos and other scaleless catfish, which do not even get lymphocystis.
Be patient. Lumps can come back later, but repeat outbreaks are usually milder.

If large lumps block the mouth or eyes and the fish is losing weight, a vet may be able to remove them under anaesthetic. This is a job for a vet, not for home surgery.

Products that help

Liquid water test kit: confirms that water quality is not adding to the stress.
Dechlorinator: for safe, regular water changes.
Hospital tank with sponge filter and heater: separates the fish from bullies and susceptible tankmates.
Extra hiding places (plants, caves, driftwood): reduce chasing and stress.
Varied, high-quality foods: support recovery.
Antibacterial or antifungal treatment: only if a secondary infection develops on an open sore.

How to prevent it

Quarantine new fish for 4–6 weeks, or longer, and watch for lumps. University fish health specialists suggest 30–60 days.
Don't buy fish with white lumps on their fins, or fish from a tank that has them.
Keep stocking levels sensible and avoid aggressive mixes.
Handle fish gently and use soft nets. Small wounds let the virus in.
Keep the water clean and stable.

Why it happened

The virus spreads by contact with infected fish or with lumps that fall off into the water. It enters through small skin injuries from fighting, spawning, parasites or rough handling. Many fish carry it without showing signs, and stress from shipping, crowding or poor water brings the lumps out. It does not appear to pass to the next generation through eggs.

When to get help

See an aquatic vet if lumps cover the gills, mouth or eyes, if the fish stops eating or loses weight, if many fish are affected, or if a lump keeps growing for months without changing, which points to a tumour instead. A vet can take a small sample to confirm what it is.

In the game

Multiple Tank Syndrome doesn't have a lymphocystis card yet. The "Quarantine tank" card is the closest match.