A fish with a swollen belly: what abdominal distension means

Quick answer

A swollen fish is a sign, not a diagnosis, and the differences between the causes are visible if you know which view to use. Look at the fish from directly above, not from the side. That single change separates the most serious cause from all the others, because scales lifting away from the body only show from above.

The other thing that decides the answer quickly is whether the swelling is symmetrical, whether the fish is still eating and swimming normally, and whether anything is coming out of the vent. Test the water before you do anything else, because a tank problem produces sick fish faster than any single disease.

Photograph the fish from directly above, every day, against the same background. Progression is what tells you which cause you are dealing with.

• Scales standing out from the body when viewed from above indicates fluid accumulation, and that is the most serious presentation.
• A smooth, even, symmetrical swelling in a female livebearer with a normal appetite is usually pregnancy.
• Constipation is common in fancy goldfish fed only dry food and often improves with soaked food and more plant matter.
• White stringy faeces alongside swelling points at internal parasites rather than at fluid.
• Wear gloves or cover any cut on your hands before putting them in an aquarium. Some fish bacteria infect people.

Decide in 60 seconds

The view that answers most of it

Look down on the fish from directly above, ideally with the tank light on and the room light off.

Scales flat. The outline is smooth. Whatever the cause, fluid is not accumulating under the skin, and the list of possibilities is much better.

Scales lifted. The outline is jagged, and individual scales stand out from the body like the scales of a pinecone. This means fluid has collected under the skin and in the body cavity, and the body's ability to control water and salt movement has broken down.

That failure is almost always downstream of something else: kidney disease, liver disease, heart failure, an overwhelming bacterial infection, or a tumour pressing on the circulation. It is why the outlook is poor and why nothing added to the water reliably reverses it. The fluid is a consequence and the cause is internal.

Also note whether the swelling is even. A symmetrical swelling of the whole abdomen behaves differently from a bulge on one side, and a one-sided bulge points at a mass, a cyst or a localised infection.

Then look at the vent. Faeces that are long, trailing and pale, or absent altogether, tell you a great deal, and so does redness or protrusion at the vent itself.

The treatable causes

Overfeeding.

The simplest and most common. A fish that has just eaten looks fuller. The distinction is that it resolves within a day and the fish is behaving completely normally. Persistent overfeeding is a water quality problem as well as a fish problem, because uneaten food decays.

Constipation.

Very common in fancy goldfish, and common in any fish fed only dry flake or pellet. Dry food swells after it is swallowed, and a fish that has taken in air at the surface while gulping floating food adds to the problem.

The signs are a swollen belly with flat scales, few or no droppings, sometimes long trailing faeces, reduced appetite and sometimes buoyancy trouble.

The usual approach is to stop feeding for a day, then soak dry food thoroughly before offering it so it expands before it goes in rather than afterwards, switch to sinking food so the fish is not gulping air at the surface, and add plant matter to the diet. Blanched, deshelled peas and other soft vegetable matter are widely used for this in goldfish; the evidence is practical rather than formal, and it is low risk in a herbivorous or omnivorous species.

Pregnancy in livebearers.

Guppies, mollies, platies and swordtails carry developing young internally. A gravid female develops a squared-off, boxy abdomen and a dark gravid spot near the vent, feeds normally and swims normally. Nothing needs doing except deciding whether you want the fry.

Egg development and egg retention in egg-layers.

A female full of eggs looks swollen and it is normal. It becomes a problem when she cannot release them, which happens when there is no suitable spawning site, no male, or the conditions are wrong. A female that has been swollen for a long time without spawning, and is starting to look unwell, needs a spawning site and often veterinary help, because retained eggs eventually break down and cause infection.

Internal parasites.

Produce a swollen abdomen combined with a fish that is losing condition elsewhere, so the back looks narrow from above while the belly is round. White, stringy, trailing faeces are the classic accompanying sign. Treatment depends on which parasite, which is why this is one to get advice on rather than to guess.

Assess through the glass. A fish that is already unwell does not need the additional stress of being netted for a look.

The serious causes

Generalised fluid accumulation.

Described above, and recognised by lifted scales. It typically comes with lethargy, loss of appetite, protruding eyes, pale gills, and sometimes red streaks in the fins.

Supportive care is what is available: excellent water quality, stable temperature, isolation from tank mates that will harass a weakened fish, and in some cases magnesium sulphate baths, which are widely used to draw fluid out osmotically. That is a common approach among keepers rather than a cure, and it is not appropriate for every species or every tank, so ask before using it.

Be honest about outlook. A fish with established fluid accumulation and lifted scales has a poor prognosis, and part of good care is recognising when the kind decision is a humane end rather than a longer course of treatment.

The cichlid bloating syndrome.

A specific and well-recognised problem in African cichlids, particularly the herbivorous rock-dwelling species from Lake Malawi. The picture is abdominal swelling, loss of appetite, white stringy faeces, rapid breathing and hiding, and it can kill within days.

It is strongly associated with feeding a diet too rich in protein to a fish whose gut is built for algae and plant material, and with stress and poor water quality. Prevention is the effective part: an appropriate high-vegetable diet for those species, restrained feeding, and good water.

Bacterial infection.

Red streaks in the fins and body, ulcers, ragged fins, pale patches and lethargy alongside the swelling. Needs proper diagnosis, because dosing blind is how filters get destroyed.

Tumours and cysts.

Usually a one-sided, slowly enlarging swelling in a fish that continues to eat and behave normally for a long time. Some are operable by a vet who sees fish; many are simply monitored.

Chronic wasting with swelling.

A fish that has slowly lost condition over months, developed a swollen abdomen, and perhaps has skin nodules or a spinal curve, may have a chronic mycobacterial infection. This group of bacteria can also infect people through cuts and grazes, causing a persistent skin infection. It is a good reason to keep any broken skin out of aquarium water, to use waterproof dressings or gloves, and to wash hands thoroughly afterwards.

A separate container with water from the display tank lets you observe, feed and treat one fish without dosing everything else.

Working through it

Test the water first. Ammonia, nitrite, nitrate, pH and temperature. If several fish are affected, the water is the answer until proved otherwise.

Photograph from above and from the side, daily. Same background, same distance, same lighting. Fish change slowly and memory is unreliable.

Watch the vent. Presence, colour and form of faeces changes the direction of the investigation immediately.

Compare with tank mates. A body shape that looks abnormal in isolation is often normal for the species, and the fish next to it is the best reference you have.

Check the feeding. How much, what type, whether it floats, whether it is soaked, and whether the species is being fed a diet that suits its gut.

Decide about isolation. A swollen fish that is being harassed, or one that needs a specific treatment, is better in a separate container filled with water from the display tank, with gentle aeration and a heater matched to the main tank. A fish that is stable and unbothered is often better left where it is, because netting and moving are stressors in themselves.

Do not stack treatments. One approach at a time, with the water tested throughout.

What commonly goes wrong

Looking only from the side. Lifted scales are invisible from the side and obvious from above, and that is the single most important observation in this whole subject.

Assuming every swollen fish has dropsy. Most do not. Pregnancy, overfeeding and constipation account for a large share of cases.

Dosing antibacterials on suspicion. They damage the biological filter, the ammonia rises, and the whole tank suffers.

Feeding dry floating food to fancy goldfish. Their compressed body shape makes them prone to both constipation and buoyancy problems, and gulping at the surface makes it worse. Soak the food and use sinking varieties.

Feeding a protein-rich diet to herbivorous cichlids. A common and avoidable cause of a fatal syndrome.

Netting a weak fish repeatedly to inspect it. Handling strips the slime coat and adds stress at the worst time.

Treating a female livebearer for illness. Pregnancy is the commonest reason a small community fish becomes suddenly rounder.

Continuing treatment indefinitely on a fish with established fluid accumulation. There comes a point where the humane option is the right one.

What never to do

Do not judge swelling from the side alone.

Do not add medication before testing the water.

Do not combine treatments, or start a second while the first is still in the tank.

Do not net and handle a weakened fish repeatedly.

Do not treat a gravid livebearer.

Do not feed dry unsoaked floating food to deep-bodied goldfish.

Do not put your hands into an aquarium with an open cut.

Do not squeeze or press a swollen fish.

Do not prolong treatment on a fish with lifted scales that is no longer eating and no longer swimming normally.

When to get help

Get advice the same day for lifted scales, refusal to eat, rapid breathing, protruding eyes, red streaks or ulcers, or swelling that has increased noticeably over a few days.

Examination out of the water is a veterinary step, done wet, briefly, and only when there is a reason. It is not something to repeat at home.

Get advice within a few days for white stringy faeces, a fish losing condition along the back while the belly enlarges, or a female that has been full of eggs for a long period without spawning.

A vet or aquatic specialist who sees fish can examine faeces and skin samples under a microscope, image the fish, and in some cases drain fluid for analysis, which distinguishes causes that look identical through the glass.

Have ready the species, how long you have had the fish, the tank volume, the full water test results with temperature, what everything in the tank is fed and how often, what has been added to the tank recently, whether other fish are affected, and photographs from above and from the side taken on consecutive days.