Goldfish with a swollen abdomen

A rounder belly than usual is a common reason people look for help, and the explanations run from trivial to grave: a fish that has eaten too much dry food at one end, and at the other dropsy, where the body fills with fluid because the organs are failing. In between sit females heavy with eggs, slow-growing internal growths, and gut parasites. A very useful thing you can do is look down on the fish from above and see whether its scales lie flat.

Telling the causes apart

Constipation comes on over a day or two, usually after heavy feeding or a new dry food. The belly is firm and rounded, the scales lie flat, and the fish is sluggish, sometimes sitting on the bottom or struggling to stay level, with faeces absent or long and stringy. Round-bodied fancies such as ryukins, fantails and pearlscales are the most prone.

A female carrying eggs is a seasonal and healthy sight. She fills out in spring or after a rise in temperature and feeding, often more on one side than the other, the males start nudging her flanks, and she eats and behaves normally with her scales flat. A pearlscale or ryukin is not bloated just because it is spherical; compare the fish with its own normal shape.

Dropsy is different in kind. The swelling is tight and symmetrical, the fish is lethargic and off its food, the eyes may bulge, and from above the scales stand out like an opened pinecone, which keepers call pineconing. It develops over days and the fish looks unwell in every way.

Slow, lumpy or lopsided swelling that has crept up over months on a fish that is otherwise eating and active points toward an internal growth or cystic kidneys. A swollen belly under a bony, wasting back, with white stringy faeces, suggests internal parasites, especially after a new fish or live food; a goldfish losing weight covers the wasting side of that picture.

How to check the scales properly

The highest-stakes observation on this page, and it is easy to get wrong through the front glass.

Look straight down from directly above, in good light, with something dark behind the fish. A healthy goldfish seen from that angle has a smooth, unbroken outline down each side. Raised scales break it into a sawtooth.

Three things worth knowing:

• Early dropsy is local. It often shows in one place first — along a single flank, or at the base of the tail — before it spreads. A fish that looks fine from the front can already show it from above.
• Some breeds mislead. Pearlscales have domed, raised-looking scales as a matter of breeding, and heavily scaled ryukins can look textured. Compare the fish against its own normal appearance, not against another fish.
• Photograph it. A picture from directly above today gives you something to compare against in three days, which is far more reliable than memory when the change is gradual.

What to do first

Test the water. Ammonia, nitrite and high nitrate all stress the kidneys and the gut, and a bloated fish in bad water will not improve until the water is fixed.

If the scales are flat, the water is clean and the fish is still interested in food, treat it as constipation. Stop feeding for one to three days; a healthy adult goldfish copes with this easily. Then offer something fibrous, such as a skinned pea or a piece of soft vegetable, and for the following weeks cut the dry food, soak pellets, or move toward sinking or gel foods. Keep the temperature stable.

The fast, done properly

Worth setting out in full, because “fast the fish” is advice everybody gives and almost nobody qualifies.

Who it is for. A well-fed adult goldfish with a full, firm belly, flat scales and a continuing interest in food. A healthy adult copes with several days without eating without difficulty.

Who it is not for. Juveniles, which are growing and have little reserve; thin fish; fish that have already stopped eating; and fish showing raised scales, which is a different problem entirely and is not helped by withholding food.

How long. One to three days, and no longer than three or four. If a fast of that length has not produced an improvement, the working diagnosis was wrong and continuing is simply starving a fish that has something else wrong with it. That is the point at which to look again at the other causes.

Breaking the fast. Something fibrous and easy: a skinned, blanched pea, or a soft vegetable from the vegetables page. Then a small meal the following day, and back to normal portions over about a week. A gut that has been empty for days does badly with a large meal.

What to change afterwards, because a fast treats the episode and not the cause: smaller portions, soaked or sinking pellets rather than floating ones, more vegetable matter, and a portion you have actually measured rather than judged. The feeding guide covers fixing the portion so it stops drifting, and constipation and stringy faeces covers the gut side in more detail.

Peas, and whether they deserve the reputation

A much-repeated goldfish remedy in the hobby, so it is worth being precise about what it does.

What is true. A skinned, blanched pea is soft, fibrous and easy to pass. Keepers use it for a mildly constipated fish and it is harmless, although no study has tested it. Skinning it matters — the skin is the indigestible part and the point of the exercise is to give the gut something it can move, not something else to struggle with.

What is not. A pea is not a treatment for dropsy, for an internal growth, for parasites, or for a buoyancy problem caused by a deformed swim bladder. It has become the reflex answer to a swollen or floating goldfish regardless of cause, and in the cases that matter it costs days.

And the plain version: most of the fish being fed peas would not have needed them. The causes are a portion that drifted upward, floating pellets swallowed with air at the surface, and a diet that is entirely dry food. Fixing those three is worth more than any remedy — the pellets page covers choosing a food a goldfish digests, and sinking versus floating covers why the pellet type matters so much for fancies.

When it is something else

Raised scales mean a vet, today if possible. Move the fish to a clean, cycled hospital tank, keep the temperature stable and the water spotless, and ask about treating the underlying infection. Some keepers and vets use Epsom salt baths to ease fluid retention, but there is no good evidence that they draw fluid off or change the outcome, and they do not treat the cause. Epsom salt is not the same substance as aquarium salt — the section below sets out the difference, which is worth getting right before you dose anything.

A female who has looked full of eggs for more than a few days, has stopped eating, or is straining and lethargic may be egg-bound. That also needs a vet. Never try to squeeze the eggs out yourself. What shows at the vent when eggs are stuck, and how to tell it from a prolapse, is on the vent page.

A long-term lumpy swelling on an otherwise happy fish is worth a veterinary opinion, if only to know what you are dealing with; many goldfish live comfortably for years with a benign growth.

Epsom salt and aquarium salt are not the same thing

A mix-up common enough to be worth its own section, because the two are used for different things and substituting one for the other simply does not work.

Epsom salt is magnesium sulphate. Some keepers and vets use baths of it for constipation, where the idea is that it relaxes the gut, and for a fluid-swollen fish, where the idea is that it eases fluid retention. There is no good evidence that it does either, and neither use has been tested in a controlled study. It does not treat an infection and it does not cure dropsy.

Aquarium salt is sodium chloride. It supports osmoregulation, reducing the work a stressed fish’s body has to do to hold its internal balance, and at controlled concentrations it is used against some parasites. The aquarium salt page covers its real but narrow role, and the fact that it is often over-used in the hobby.

Neither should be dosed by guesswork on a fish that is already struggling. Both depend on the real volume of water, which is usually smaller than the tank is sold as, and on whether the fish is in the display tank or a bath. Follow a specific protocol from a reliable source, or ask a vet.

What a vet can actually do

Worth knowing, because people often do not call one on the assumption that nothing can be done for a fish.

• Identify what is actually happening. A swollen abdomen has half a dozen causes with completely different outlooks, and distinguishing an egg-bound female from a tumour from kidney failure is not something you can do by looking. Imaging and a sample of the fluid change the answer.
• Inject antibiotics. This is the significant one. A bacterial infection behind dropsy is largely out of reach of anything dissolved in the tank, and in-water antibiotics are both weak at the site and hard on the filter. Injectable treatment from a vet is a genuinely different proposition.
• Draw off fluid. Aspiration can relieve pressure on the organs and buy a fish time and comfort.
• Say when to stop. Which is a service too.

Finding an aquatic vet covers how to locate one — there are more than people expect, and many will advise by phone or email if seeing the fish is impractical.

When to stop treating

An uncomfortable section and a necessary one, because this symptom includes the cases where the kindest decision is not to keep going.

A fish with fully raised scales, refusing food for days, unable to hold itself level and visibly labouring is not a fish in the early stages of something treatable. Continuing to add medications at that point usually extends the period during which it is unwell rather than changing the outcome.

Humane euthanasia covers how that is done properly and painlessly, which is a small kindness worth knowing about before you need it. Goldfish welfare is the wider framing: the question is not whether the fish could survive another week, but what that week would be like for it.

There is no rule for where the line is, and the decision is a keeper’s to make. Being told plainly that it is a legitimate decision is usually the part that helps.

Preventing it

Most bloating is fed into the fish. Small meals, one to three times a day, eaten within a minute or two, with regular vegetables and less dry food, keep the gut moving; our guide to overfeeding goldfish explains how little a goldfish needs. Keep ammonia and nitrite at zero and nitrate low with weekly water changes, since poor water and chronic stress make dropsy more likely, though viruses, parasites and tumours can cause it too. Quarantine new fish to keep parasites out, and if a fish starts floating after meals, treat that as the early warning it is.