Lumps, growths and tumours on a goldfish

A lump on a goldfish sets off an immediate train of thought that ends at the worst answer. It is worth saying at the outset that many lumps are not tumours, that several of the common ones are harmless, and that at least two of the harmless ones are routinely medicated by keepers who did not know what they were looking at.

Sorting them out is mostly a matter of three observations: where the growth is, what it looks like, and how fast it is changing.

Where it is

On a fin or on the surface of the skin. Carp pox, lymphocystis and the attachment sites of parasites all sit on the outside. They are visible in profile against the light and the surface texture is the diagnostic feature.

Under the skin. Cysts, abscesses, encysted flukes and most tumours are beneath the surface, so the skin over them looks normal or slightly stretched rather than textured.

Inside the body. A swelling that distorts the body shape without a discrete lump is a different problem and belongs to the swollen abdomen page — internal growths, egg retention, fluid accumulation and organ enlargement all present that way.

What it looks like

This is where most of the identification happens.

Appearance	Most likely
Smooth, waxy, translucent, like dripped candle wax	Carp pox
Cauliflower or raspberry texture, clustered, usually on fins	Lymphocystis
A white or greenish thread from a red, inflamed spot	Anchor worm
A flat translucent disc that moves about on the fish	Fish louse
Soft or firm swelling under normal-looking skin	Cyst or abscess
Firm, irregular mass, no inflammation, slow	Tumour
Small raised black or yellow spots, pond fish	Encysted larval flukes
Raised tissue on the head, symmetrical, in an oranda or ranchu	The wen — usually harmless

The base is the tell that separates the two big groups. A red, inflamed point of attachment means something is living there. A growth arising from the fish’s own tissue does not inflame its surroundings unless it has become infected or has started to break down.

First, is it the breed?

A common false alarm on this page, and the easiest to resolve.

Orandas, ranchu, lionheads and pompoms grow raised tissue on the head or around the nostrils as the defining feature of the breed. It starts as a roughened, slightly swollen look at a few months old and thickens through the first year or two. It is symmetrical, it develops evenly, and the fish is perfectly well throughout.

Wen care covers the tissue properly — how it develops, what it costs the fish, and why almost all of looking after one is water quality.

Three things are worth watching on a wen that is otherwise normal:

• Growth over the eyes, which can impair vision and occasionally needs a vet to trim it. This is done under anaesthetic and is a routine procedure.
• Redness, fraying or fuzziness in the folds, which is an infection in a warm damp crevice rather than growth. That needs the attention any fungal or bacterial patch would.
• A change in shape, such as going lumpy on one side, ulcerating or interfering with swimming. A small published series of overgrown oranda wens found a tumour in three of the four, so a wen that changes is worth a vet’s opinion, and a vet who trims one can send a sample to be checked.

Carp pox: the one to leave alone

If the growth looks like candle wax that dripped onto the fish and set, this is probably what you are looking at.

The name comes from a cyprinid herpesvirus, CyHV-1, that is known only in common carp and koi; in goldfish the cause is less clear, and one reported goldfish papilloma turned out not to be viral. Goldfish carry a different herpesvirus of their own, CyHV-2, which makes them seriously ill with pale gills, swelling and heavy losses; that is a sick-fish problem for a vet, not a lump. The important facts about the growths are unusual enough to state plainly:

• It is usually harmless. The fish is not ill and behaves normally throughout, though a growth can become infected, and one that lasts through summer, enlarges or ulcerates needs a vet.
• It is seasonal. Growths appear as the water cools and recede as it warms, which is why it turns up in ponds in autumn and spring, and why people report it “coming back”.
• It is untreatable. No medication acts on it, and surgery does not help.
• It may be common, and in carp the virus is often present in a group without producing visible growths in most of them.

So the practical value of recognising it is negative: it is the knowledge that stops you doing something. A fish with carp pox that is put through a course of antiparasitic or antibacterial treatment has been given the full cost of the medication and none of the benefit, which is the pattern the medications page is built around.

The lumps that are parasites

These are the ones worth identifying precisely, because unlike almost everything else on this page they respond to treatment.

Anchor worm. Not a worm but a parasitic copepod. It embeds its head into the fish’s muscle and leaves a thread-like body a few millimetres long protruding, usually whitish or greenish, sometimes with two small egg sacs at the free end. The attachment point is reddened and inflamed, and the fish often rubs against surfaces, which is the flashing the earlier pages describe.

Fish lice. Flat, round, translucent, up to about 5 mm across, and — the giveaway — they move around on the fish, so if you watch a suspected louse for a minute you will usually see it reposition. They leave small red feeding marks.

Both arrive from outside: with a new fish, with plants, or with live food collected from water containing other fish. Neither appears spontaneously in a closed tank, which makes both of them arguments for quarantine.

Treatment is a specific one aimed at crustacean parasites rather than a general remedy, and the attachment wound needs attention afterwards. Manual removal of anchor worm is standard practice and is easy to do badly: pulling on the visible part can leave the anchor buried and tear the surrounding tissue. It is worth having someone show you once, and the anchor worm and fish lice page covers the life cycle behind the treatment.

Tumours

Goldfish get tumours. They are among the better-documented tumour-bearing pet fish, partly because they live a long time and partly because a fish in a glass box gets looked at closely.

What a tumour typically looks like: a firm, irregular mass that grows slowly over months, arises from the fish’s own tissue rather than sitting on it, has no inflamed base, and does not resolve. Some are benign and some are not, and nothing about the external appearance reliably separates the two.

What can be done about it is more than most people expect. Fish are routinely anaesthetised, and the surgical removal of an external mass is established aquatic veterinary practice. Whether it is appropriate depends on the type, the location, the fish’s age and condition, and what imaging shows — which is a conversation with a vet who sees fish rather than a question this page can answer.

What is not worth doing is guessing. A growth that has been stable for a year on a well fish needs watching, not a decision.

Rate of change is the measurement that matters

Everything above describes appearance, which is useful. Rate of change is more useful, and it is the observation almost nobody records.

• Appeared within days, inflamed base — parasite, injury or infection far more often than a tumour, although a fast-growing or breaking-down tumour is possible.
• Grew over two or three weeks, then stopped or softened — cyst or abscess.
• Enlarging steadily, month after month — the picture that warrants a vet.
• Unchanged for months or years — the best case, and the one where leaving it alone is the correct treatment.
• Comes and goes with the seasons — carp pox.

Which is why a very useful thing a keeper can do here costs two minutes a month:

Photograph it, dated, with something in frame for scale. A ruler against the glass, a known decoration, anything. Human memory is spectacularly unreliable about slow change, and a keeper watching a lump daily will either convince themselves it is growing or fail to notice that it is. A vet’s first question will be how fast it has changed, and a set of dated photographs answers it in a way that no amount of description can.

The interventions to resist

Do not cut, lance or squeeze it. You cannot tell what is inside from outside, a cut will bleed, heal slowly in water and invite infection, and the open wound you create is a far more immediate problem than the lump was. This applies to cysts that look ready to pop as much as to anything else.

Do not treat blind. A course aimed at the wrong thing costs the fish the stress and the tank its filter bacteria, and buys nothing. If the growth is waxy or cauliflower-textured, no available medication acts on it at all.

Do not run treatments consecutively hoping one will work. The medications page sets out why that is the most expensive mistake in fishkeeping.

Do not isolate a long-established fish on suspicion. If the lump were transmissible the tank has already been exposed; moving the fish adds stress without changing the exposure. Quarantine belongs at the point of entry.

When it is urgent

Most of this page counsels patience. These are the exceptions:

• Bleeding, or an open ulcerated surface. That is now a wound as well as a growth, and the ulcers page applies.
• Interference with function — a growth that stops the fish swimming level, reaching food, closing its mouth, or moving a gill cover.
• Rapid enlargement over days rather than months.
• A lump plus systemic signs: weight loss, going off food, sitting on the bottom, laboured breathing. The lump may not be the main problem.

Living with a lump

Worth ending on, because it is the outcome for a great many of these fish.

Goldfish frequently live for years with a growth that never does anything. A stable lump on a fish that eats, swims normally, holds its fins open and behaves like itself is compatible with a full life, and the healthy baseline page describes what you are checking against each time you look.

The question that matters is not whether the fish has a lump but whether the lump is costing it anything. When it eventually does — when a fish can no longer feed, swim or breathe properly and there is nothing to be done — a vet can end its life quickly and painlessly, and that is a better answer than waiting. Putting a goldfish to sleep humanely covers how to judge that point. Most fish on this page never reach that point.