Dropsy in fish: pinecone scales, what it means, and when to let go

Dropsy is fluid building up inside a fish's body, making it swell until the scales stick out like a pinecone. Move the fish to a hospital tank and test your main tank's water today, because most fish with full pinecone dropsy die, and the cause may affect your other fish too.

Dropsy isn't one disease. It's a sign. The fluid collects in the belly (called ascites) or under the skin and scales (called oedema). The Merck Veterinary Manual and a veterinary case report on a goldfish both describe "dropsy" and "pinecone disease" as names for signs, not specific diseases. Fresh water constantly soaks into a fish through its gills and skin, and the kidneys pump it back out all day. When the kidneys, liver or heart fail, or the gills or skin are badly damaged, the fish can't get rid of the water fast enough, and it swells.

How to identify it

A belly that looks swollen or rounder than usual, without the fish eating more.
Scales sticking out from the body, like a pinecone, when you look from above.
Bulging eyes (pop-eye), a swollen or red vent.
Pale gills, stringy poop, less active, less interested in food, hanging alone.

Easily confused with:

A female full of eggs or a pregnant livebearer: round, but scales lie flat and the fish acts normally.
Overeating or constipation: swollen after feeding, goes down in a day or two (see overfeeding).
Tumours or internal worms: a lump or one-sided swelling.
Bloat in cichlids: swollen belly with appetite loss, often linked to diet and gut problems; may respond to treatment if caught early.

When it's a problem

Early: a slightly swollen belly, slightly bulging eyes, scales only slightly raised, fish still eating. Treatment is worth trying, and these cases have a better chance.
Serious: clear pinecone scales and a badly swollen body. By this point the organs are usually badly damaged, and most fish die even with treatment.
Emergency: several swollen fish at once. That pattern suggests an infection or a toxin affecting the tank. Call a vet.
Time to let go: the fish can't swim upright, lies on the bottom, or has stopped eating. It's suffering, and humane euthanasia (below) may be the kindest choice.
Normal, don't worry: a round female with flat scales who acts normally, or a fish that's swollen after a big meal and back to normal in a day or two.

If you lose a fish to dropsy, that doesn't mean you did something wrong. Dropsy often hides until it's late.

What to do right now

Move the swollen fish to a hospital tank or a clean container with tank water, a heater and an air stone, if you can. If the cause is infectious, this protects the others.
Test the main tank: ammonia, nitrite, nitrate, pH, temperature. Dropsy is often a warning about the tank, not only the fish.
Do a 25–30% water change in the main tank, and watch the other fish for the next two weeks.
Look closely at the fish and decide, using the list above, whether to treat or to let it go.
If you have several swollen fish, call a vet.

What to do next

The honest outlook

Most fish with full pinecone dropsy die, even with treatment. In the goldfish case report, the fish improved for a while under a vet's care with injected drugs, then relapsed and was euthanised. Early cases, caught when the belly is only slightly swollen, have a better chance.

What you can try for early cases

Clean, warm, stable water in a hospital tank, at the middle of the species' preferred range. Change 25% every day or two.
Salt. Many keepers add 1–3 g per litre of aquarium salt or plain non-iodized salt. The idea is to reduce how much water soaks into the fish, easing the load on the kidneys. Evidence is limited, and it won't help if the cause is liver or heart failure. Some fish (many catfish, plecos and loaches, and soft-water species) are thought to tolerate salt poorly; experts disagree, so be cautious with them.
Epsom salt (magnesium sulfate) is a common hobby remedy for swelling. It's widely used, but there's little evidence it cures dropsy. If you try it, use it in the hospital tank only.
Antibiotics. If a bacterial infection is the cause, an antibiotic can help, especially given in food while the fish still eats. Many countries require a vet prescription for fish antibiotics. A vet can choose a drug that's safe for already-damaged kidneys. Don't guess; some antibiotics are hard on the kidneys. Follow the label or your vet's directions, remove activated carbon during treatment, and finish the course.

Give treatment about a week. If the swelling gets worse or the fish stops eating, it's time to think about letting it go.

Humane euthanasia with clove oil

If the fish is suffering and not improving, euthanasia is a kind and responsible choice. The best option is a vet, who can use an anaesthetic overdose. If that isn't possible, clove oil is the most widely recommended home method. This method follows the RSPCA (Australia) guidance:

Put some tank water in a clean container that has never had soap in it. Move the fish into it gently.
Use about 0.4 ml of clove oil per litre of water (about 1.5 ml per US gallon). Clove oil from a pharmacy or health shop works; a standardised product is better because strength varies.
Mix the clove oil with a little warm water first, in a small jar with a lid, and shake hard. Clove oil doesn't mix with water on its own, and blobs of oil can burn the fish.
Add the mixture slowly over about 5 minutes, not all at once. Adding it fast can excite the fish. As the dose builds, the fish loses consciousness, then stops breathing.
Leave the fish in the solution for at least 30 minutes after its gills stop moving.
Check for death: no gill movement for 30 minutes, and no eye reflex when you touch the surface of the eye gently.
Only then, as a final step to make certain, place the fish in the freezer until fully frozen.

Never flush a live fish, put a live fish in the freezer, drop it in boiling water, or leave it out of water to suffocate. The RSPCA lists all of these as unacceptable. Physical methods (a hard blow to the head, or decapitation) are used by trained people, but are hard to do correctly without experience.

For disposal, the RSPCA suggests burying the body at least 30 cm (1 foot) deep, well away from waterways, or wrapping it for household rubbish. Don't flush it or put it in natural water.

Products that help

Hospital tank with a heater and air stone: separates a sick fish so you can treat it and protect the others.
Test kit for ammonia, nitrite, nitrate and pH: checks whether the main tank's water is part of the cause.
Aquarium salt (or plain non-iodized salt): may ease the load on the kidneys in early cases. Evidence is limited; be cautious with catfish, plecos, loaches and soft-water fish.
Epsom salt: a common hobby remedy for swelling, with little evidence. Hospital tank only.
Antibiotic food or medicine from a vet: for a bacterial cause, while the fish still eats. Often needs a prescription.
Clove oil, ideally a standardised product: for humane euthanasia at home when a vet isn't possible.

Before trouble starts, keep a small hospital tank, a test kit and a bottle of clove oil on hand, so you're not deciding in a rush.

How to prevent it

Keep water clean and stable, with weekly tests and regular water changes.
Feed a varied diet in sensible amounts.
Quarantine new fish.
Check your fish from above every day or two. Spotting a slightly swollen belly early gives you the best chance.

Why it happened

Bacterial infection of the internal organs, often Aeromonas bacteria, usually in a fish already weakened by stress or poor water. The Merck pet owner page links classic dropsy to Aeromonas, which needs lab testing to confirm.
Kidney disease from parasites, tumours or old age. Merck describes a parasite-caused kidney dropsy in pond goldfish with "no practical treatment".
Long-term poor water quality (ammonia, nitrite, high nitrate) that wears down the organs.
Liver damage from poor diet, chronic overfeeding, or toxins.

When to get help

Call a vet who sees fish if more than one fish swells, if the fish is valuable to you, or if you want humane euthanasia done for you. A vet can sample the fluid, culture bacteria, and prescribe the right drug. If you lose a fish, a vet can also do a post-mortem to tell you whether the rest of the tank is at risk. Honest limit: for advanced dropsy, the most useful thing a vet often offers is a peaceful end.

In the game

In Multiple Tank Syndrome, the Dropsy card is blunt: "one fish swells up. It dies; nothing saves it." No product card helps. That's harsh, but it reflects how often advanced dropsy ends in real tanks.