The short answer
Dropsy isn’t one disease — it’s a serious symptom where a fish swells up and its scales stick out like a pinecone, usually caused by fluid building up inside the body. It often signals internal organ trouble and the prognosis is generally poor. The safe response is to isolate the fish, check your water immediately, keep it comfortable, and seek advice from a vet or experienced keeper. Please treat this as general information, not a diagnosis.
How to recognise it
The classic sign is a bloated body with raised scales — viewed from above, the fish looks swollen with scales flaring outward (often described as “pineconing”). Other signs can include bulging eyes, loss of appetite, lethargy and clamped fins. By the time pineconing is obvious, the condition is usually advanced.
Not all bloating is dropsy — constipation and swim bladder issues cause swelling too — but the raised, pineconed scales are the distinguishing feature.
What to do
Because dropsy is serious, the sensible steps are to isolate the affected fish in a hospital tank so you can observe it and protect the rest of your fish, keep the water clean, warm and stable, and consult a vet or experienced fishkeeper about the underlying cause. For setting up isolation, see how do I set up a hospital tank? Avoid guessing at strong medications — dosing blind can do more harm than good.
Be prepared that even with good care, dropsy often can’t be reversed. Keeping the fish comfortable and reducing suffering is a valid and humane goal.
Prevention
The best defence is a stable, cycled tank with clean water, sensible stocking and quarantined new fish. For the wider picture, see how do I prevent fish disease?