Dropsy is not a disease on its own. It is a sign that fluid is building up inside a fish’s body, usually because the kidneys or other organs are failing. A bacterial infection is one common cause, but not the only one.
Signs of dropsy
- A swollen belly that looks bloated from the side.
- Raised scales that stick out, giving a “pinecone” look when seen from above.
- Bulging eyes and pale gills.
- Lethargy and loss of appetite, often hanging near the surface or the bottom.
What causes it
- Bacterial infection. The Merck Veterinary Manual describes Aeromonas and Pseudomonas bacteria causing internal bleeding and septicemia, more often in freshwater fish (Bacterial Diseases of Fish).
- Organ damage from long-term poor water quality.
- Other causes such as internal parasites, tumors or old age.
Is dropsy contagious?
Dropsy itself isn’t, but a bacterial infection behind it can spread. Move the swollen fish to a hospital tank and watch the rest of the tank closely.
What you can do
- Move the fish to a hospital tank with clean, stable water.
- Test the main tank. Ammonia and nitrite should be zero; do a partial water change if they aren’t.
- If a bacterial infection is likely, ask an aquatic veterinarian which antibiotic fits; a broad product such as Fish Mox (fish amoxicillin) is labeled for common bacterial infections. Follow the label.
- Feed lightly with good-quality food, and remove anything uneaten.
Be realistic: once the scales stand out like a pinecone, many fish don’t recover, because the organs are already badly affected. Early action gives the best chance.
When to call a veterinarian
Ask an aquatic veterinarian if more than one fish swells up, or if a valuable fish shows early signs. A vet can find the cause. If a fish is suffering and not improving, a vet can also advise on humane euthanasia, which Merck covers in Management of Aquarium Fish.
Preventing dropsy
- Keep the water clean with regular testing and partial water changes.
- Don’t overfeed or overcrowd.
- Quarantine new fish before they join the main tank.
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Fish antibiotics are for ornamental aquarium fish only: not for human use and not for food-producing animals. This guide is general information, not veterinary advice; for valuable or persistently sick fish, ask an aquatic veterinarian. Last reviewed: November 2026.