Bloated fish: constipation, dropsy or eggs? How to tell and what to do

A bloated fish is most often constipated from overfeeding, and the first fix is to stop feeding for two to three days. The key check is the scales: if they lie flat, it's probably constipation or a female carrying eggs; if they stick out like a pinecone when seen from above, it's dropsy, which is far more serious.

A swollen belly is a sign, not a diagnosis. Fish vets list many causes: a full gut, eggs or fry, fluid from organ failure or infection, and tumours. This guide helps you sort out which one you're likely dealing with.

How to identify it

What you see	Most likely
Round belly, scales flat, still eating, few or no droppings, or long stringy droppings	Constipation
Round belly in a female, scales flat, eating and acting normally; a dark "gravid spot" near the vent in livebearers	Eggs or fry (normal)
Swelling with scales raised like a pinecone when viewed from above; often lethargic, not eating, sometimes bulging eyes	Dropsy
Lump or swelling on one side only, growing slowly over weeks or months; fish otherwise fairly normal	Tumour or cyst
Swollen after a big meal, floating or tipping, struggling to stay level	Overeating, possibly with a swim bladder problem

Look from above with a torch. Raised scales are much easier to see from the top than from the side.

Livebearers and egg-layers

Female guppies, mollies, platies and swordtails are pregnant much of the time and can look huge just before giving birth. A boxy, squared-off belly and a dark spot near the vent mean fry are close. Egg-laying females (tetras, cichlids, goldfish) swell with eggs before spawning. Occasionally a female can't release her eggs ("egg-bound"); she stays swollen for weeks and becomes unwell. That needs a vet.

When it's a problem

Normal: a rounder belly right after feeding; females full of eggs or fry who eat and swim normally.

Early: swollen with flat scales, still active, few droppings. Fast and change the diet.

Serious: no improvement after 3–4 days of fasting, the fish stops eating, or it struggles to swim.

Emergency: pinecone scales, bulging eyes, red streaks or sores, or the fish lying on the bottom breathing hard. Go to dropsy.

What to do right now

Look from above for raised scales.
Stop feeding the affected fish (or the whole tank) for 2–3 days. Healthy adult tropical fish manage this easily.
Test the water for ammonia, nitrite and nitrate. Poor water makes every one of these problems worse.
Do a 25–30% water change with dechlorinated, temperature-matched water.
Check the temperature. Cold water slows digestion. Keep tropical fish in their normal range, usually 24–27°C (75–80°F).
Move a heavily pregnant livebearer to a breeding box or a planted area so the fry have cover, but don't keep her in a tiny box for more than a day or two.

What to do next

Constipation

Day 1–3: fast. Many cases clear up with nothing else.

Day 3–4: feed a small amount of thawed frozen or live daphnia. Daphnia have a tough outer shell that works like fibre and helps move things along. Soak dry foods in tank water before feeding from now on.

The pea myth: you'll read everywhere that a shelled, cooked pea cures constipation. For plant-eating fish such as goldfish, mollies and many plecos, a little blanched, skinned pea or other vegetable is fine. But carnivores such as bettas and most cichlids aren't built to digest plant matter, so peas are a poor fix for them. For carnivores, use daphnia or brine shrimp instead.

Epsom salt (with care): Epsom salt is magnesium sulphate, not aquarium salt. It acts as a mild laxative and is often used for constipation. A commonly published hospital-tank dose is about 1 level teaspoon per 19 litres (5 US gallons). Dissolve it in tank water first, add it to a hospital tank rather than your main tank, and use it for no more than a few days. Hobby sources give very different doses for stronger short baths, so if you're new, stick to the gentle hospital-tank dose. Don't use Epsom salt as a cure for dropsy; it can ease swelling but it doesn't treat the cause.

Eggs or fry

Leave her alone. Keep the water clean and give her plants to hide in. If an egg-laying female stays swollen for several weeks and starts to look unwell, she may be egg-bound. Vets can treat this.

Dropsy

Dropsy means fluid has built up inside the fish, usually because kidneys or other organs are failing, often from a bacterial infection. Move the fish to a hospital tank and follow the dropsy guide. Antibiotics are sometimes used, and many countries require a vet prescription for them. The outlook is poor once the scales have lifted, especially in small fish, and humane euthanasia is sometimes the kindest choice.

Tumours

Fish can get cancers and cysts, especially goldfish and older fish. A slow, one-sided lump in a fish that keeps eating may not need treatment for a long time. Some tumours can be removed by a vet.

Cautions

If you medicate, remove carbon from the filter and finish the full course. Plecos, loaches, scaleless catfish, shrimp and snails are sensitive to many medicines, so treat in a separate tank where you can.

Products that help

Frozen or live daphnia: a fibre-rich food that helps constipated fish pass waste.
Epsom salt (pure magnesium sulphate, no scent or additives): a mild laxative for constipation, used in a hospital tank.
Hospital tank with heater and sponge filter: lets you treat one fish without dosing the main tank.
Liquid test kit: rules water quality in or out.
Varied, quality foods sized for your fish: prevents most constipation.
Breeding box or dense plants: for pregnant livebearers and their fry.

How to prevent it

Feed less than you think. Most constipation is overfeeding. See overfeeding.
Give one fasting day a week.
Vary the diet: pellets, frozen foods and, for plant-eaters, vegetables.
Soak dry pellets and freeze-dried foods before feeding.
Keep the temperature right for the species.
Keep the water clean with weekly partial changes.

Why it happened

Most bloating is food: too much, too dry, or the wrong kind for the fish's gut. Dropsy is different: it's a sign that the fish's internal organs can't control water balance, often after long-term stress or poor water lets an infection take hold.

When to get help

Get help if the scales lift, if the swelling doesn't improve after 4–5 days of fasting and daphnia, if a female stays swollen for weeks, or if the swelling is a growing lump. An aquatic vet can use imaging or a fluid sample to find the cause.

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