Fish lumps, spots and growths: telling them apart and logging one

Quick answer

Almost everything that appears as a lump, spot or growth on a fish looks like several other things, and the treatments for those things are different and mutually incompatible. Guessing and dosing is the most common way owners turn one problem into two.

Most of the diagnosis happens by looking carefully at the same fish on several days, in the same light.

Two things do more for a fish with a lump than any bottle. Test the water, because a large proportion of skin problems in fish are water quality problems wearing a costume. And document the lump properly, because whether it grows, spreads, moves, or affects other fish is what actually identifies it.

There is one common finding that is not a disease at all. Male goldfish and koi develop small white raised dots on the gill covers and pectoral fins during the breeding season, and every year those get treated as white spot by owners who have not seen them before.

• Test ammonia, nitrite, nitrate, pH and temperature before you do anything else.
• Photograph the lump against a ruler held outside the glass, same angle and light, dated.
• Small salt-grain white dots on the gill covers of a male goldfish in spring are breeding tubercles, not ich.
• Do not medicate blind. Many treatments harm invertebrates, plants, the biofilter or scaleless fish.
• A nodular growth with progressive wasting needs an aquatic vet and, because of the zoonotic risk, gloves for tank work.

Decide in 60 seconds

Test the water first, every time

The skin and gills of a fish are in permanent contact with the water. Ammonia and nitrite damage those tissues directly, and damaged tissue is where fungus, bacteria and parasites establish.

So an enormous share of what look like primary skin diseases are secondary to a chemistry problem, and treating the visible lesion while leaving the cause in place fails.

Before anything else, measure ammonia, nitrite, nitrate, pH and temperature, and write the readings down. If ammonia or nitrite is detectable at all, that is your first problem, and the response is water changes and aeration rather than medication.

Also ask what changed. A new fish, a filter clean, a dead fish that was not found, a change of food, a heater failure, a power cut, or a large water change with mismatched temperature all destabilise a tank.

What the common findings actually look like

White spot.

Discrete white dots the size of grains of salt, scattered across the body and fins rather than confined to one area. The fish flashes and rubs. The parasite lives under the outer layer of skin, so it cannot be wiped off, and treatment targets the free-swimming stage rather than the cyst on the fish. Progresses over days and spreads through a tank.

Breeding tubercles.

Also small, white and raised, and this is the one that gets misdiagnosed every spring. The distinguishing features are location and distribution: tubercles appear on the gill covers and along the leading rays of the pectoral fins, in neat rows, on mature male goldfish, koi and some other cyprinids, during the breeding season. They are not scattered over the body, the fish is not flashing, and no other fish in the tank develops the same pattern in the same way. They are a normal secondary sexual characteristic and need nothing.

Velvet.

Not discrete dots but a fine dusting, gold or rusty, best seen with a torch held at an angle. The fish clamps its fins, flashes, and often breathes rapidly because the gills are affected. Progresses quickly.

Fungus.

A cottony or woolly grey-white tuft, usually growing out of an existing wound, a damaged fin, or an area of skin injury. Fungus in an aquarium is nearly always secondary. Finding the primary problem, which is usually an injury or a water quality issue, matters more than treating the fluff.

Columnaris.

Frequently mistaken for fungus. Typically a pale grey or white patch, classically saddle-shaped across the back, sometimes around the mouth, with a flatter, more slimy appearance than true fungus. It moves fast and is often fatal without prompt treatment, and it is favoured by warm water and poor conditions.

Epistylis.

A protozoan colony that looks very like white spot but sits on a raised base, often with a reddened or bloody centre. It is associated with poor water quality and organic load, and it does not respond to white spot treatments. The clue is the base of the lesion.

Lymphocystis.

A viral condition producing cauliflower-like clusters of white, grey or pink nodules, most often on the fins but sometimes on the body. It looks alarming and is generally self-limiting, resolving over weeks to months. There is no specific treatment, medicating does nothing, and the useful response is to reduce stress and maintain good water quality.

Fish pox.

Smooth, waxy, raised plaques, most often on koi and goldfish, that look like candle wax dripped on the fish. Associated with cool water and often fading as the water warms.

Ulcers.

An open sore, typically with a red centre and a pale margin, and often with the scales lifted around it. Bacterial and serious, and common in pond fish. Needs prompt attention.

Anchor worm.

A visible thread or Y-shaped filament protruding from the fish with an inflamed red base where it is embedded. It is a parasitic crustacean. You can see it with the naked eye, which distinguishes it immediately.

Fish lice.

Flat, round, translucent discs, several millimetres across, that move about on the fish. Also visible, also crustacean.

Black or yellow spots.

Small dark or yellow points under the skin, usually in pond fish, from the larval stage of a fluke that needs snails and birds to complete its life cycle. Usually of little consequence to the fish and not treatable directly.

Tumours.

Firm nodules that grow slowly over months, do not respond to any treatment, do not spread to other fish, and may distort the body shape. Common in older goldfish and koi. Some are operable by an aquatic vet, many are not, and the decision turns on whether the fish can still eat, swim and breathe.

Mycobacteriosis.

Chronic and cumulative. Nodules, ulcers that will not heal, scale loss, a bulging eye, spinal curvature, and progressive thinness despite eating. Fish are lost one at a time over months. Generally untreatable and transmissible to people through broken skin.

Not a lump at all.

A generalised swelling with the scales standing out is dropsy, a sign of fluid accumulation rather than a growth. A bilateral swelling in a female during the breeding season may simply be eggs. A one-sided bulge behind the head can be an internal problem rather than a skin one.

Look at where the lesion sits, what its base looks like, and whether the surrounding skin and scales are normal.

How to log a lump properly

This is the part that converts guesswork into something a vet or an experienced keeper can act on.

Photograph it against a scale.

Hold a ruler flat against the outside of the glass in the same plane as the fish, so the photograph carries a size reference. Turn off the room lights and use the tank light only, or use consistent lighting either way, and take the shot square-on rather than at an angle.

Same angle, same light, same day of the week.

Consistency is what makes two photographs comparable. A lump photographed from a different angle in different light looks like a different lump.

Record the details in words.

Where it is, precisely. Which side, which fin, how far from the eye or the base of the tail. Its colour. Whether it is raised, flat or ulcerated. Whether it has a defined edge. Whether the base is red. Whether the scales over it are normal, missing or lifted.

Note whether it moves.

A parasite that changes position between observations identifies itself.

Note the fish's behaviour.

Appetite, position in the tank, breathing rate, whether it is flashing or rubbing, whether it is holding its fins clamped, and whether it is being harassed by tank mates.

Note the other fish.

Something that appears on several fish over a short period is infectious. Something on one fish for months, while everything else stays well, usually is not.

Note the water tests with the same dates.

Note the timeline.

When you first saw it, and what the tank was doing at the time.

A dated log with water readings alongside the photographs is worth more to a vet than any description from memory.

The three questions that narrow it fastest

Is it spreading to other fish?

Yes points at parasites and infectious bacterial disease. No, over weeks, points at a tumour, lymphocystis, an old injury or an individual problem.

Is it growing quickly or slowly?

Days means an active infection. Months means a growth.

Does it sit on the surface, or in the skin?

Something you could imagine brushing off, cotton wool or a moving disc, is on the surface. Something clearly within the tissue, with normal skin over it, is not, and no bath treatment will reach it.

Add a fourth for pond fish in particular: has the season changed. Several conditions, including fish pox and breeding tubercles, are seasonal, and cold water suppresses the fish's immune response so spring is when problems become visible.

Treating, and why blind treatment causes harm

Medication is not neutral. Many aquarium treatments are toxic to shrimp, snails and other invertebrates. Some damage plants. Several harm scaleless fish such as loaches, some catfish and eels, which have less protection and absorb more. Copper-based products in particular need care.

Medication harms the filter. Antibacterial and some antiparasitic products kill nitrifying bacteria, so a treated tank can develop an ammonia spike partway through a course, which stresses the fish further.

Carbon removes medication. If you run activated carbon, take it out before dosing or you will treat the carbon rather than the fish.

Dose by actual water volume. Substrate, rock and decoration displace a substantial amount, so the number on the tank is not the number in the tank. Overdosing is common for exactly this reason.

Salt is not universal. It helps in some situations and harms plants, some invertebrates and some fish species. It is a tool, not a default.

A hospital tank is often better than treating the display. It uses far less medication, protects invertebrates and plants, and lets you observe the affected fish properly. It needs its own heater, filtration and matched water.

Do not stack treatments. Combining products, or starting a second one because the first has not worked in two days, is a common route to a wiped-out tank.

What never to do

Do not medicate before testing the water.

Do not try to pull, scrape, cut or squeeze anything off a fish. The slime coat is the fish's primary barrier and breaking it invites infection.

Do not treat breeding tubercles.

Do not net and handle a fish repeatedly to inspect a lump. Photograph it in the water instead.

Do not combine medications, or start a new one before the previous course has finished and been removed.

Do not siphon a tank by mouth, ever.

Do not put your hands in a tank with a suspected mycobacteriosis case, or with any tank if you have a cut, without waterproof gloves.

Do not move fish or equipment from a tank with an unidentified spreading problem into another tank.

Do not assume a fish with a lump is doomed. Lymphocystis resolves, breeding tubercles are normal, and many parasites are entirely treatable once correctly identified.

When to get help

Contact an aquatic vet or an experienced specialist urgently for an open ulcer, for a pale lesion that is visibly spreading over hours to days, for several fish becoming affected at once, or for any fish that is struggling to breathe.

Contact one for a growth that is enlarging over weeks in a fish that is also losing weight, and treat that tank with gloves in the meantime.

Contact one for anything you cannot identify confidently, before you medicate rather than after. Identification changes the treatment completely, and a wrong treatment costs time the fish may not have.

Intact skin, fins carried open, normal colour and steady breathing. That is the baseline your photographs are being compared against.

Bring the dated photographs with the ruler in frame, the water test results with matching dates, a list of every fish in the system and which are affected, the tank's age and history, recent additions and changes, and any treatment already given including the product name and how much was used. Treatment already administered changes what can be done next, so be complete about it.