Hi,
Dropsy in a Flowerhorn is not a disease itself but a symptom of a serious internal problem, usually caused by a bacterial infection, kidney failure, poor water quality, or severe stress. Early treatment gives the best chance of recovery.
Treatment Steps:

1. Isolate the fish in a quarantine tank if possible.
2. Improve water quality immediately by performing a 30–50% water change and ensuring:
   • Ammonia: 0 ppm
   • Nitrite: 0 ppm
   • Nitrate: Below 20 ppm
3. Increase the water temperature gradually to 28–30°C with good aeration.
4. Add aquarium salt as per the manufacturer’s recommended dosage (avoid overdosing).
5. Start a broad-spectrum antibacterial medication suitable for treating internal bacterial infections, following the product instructions carefully.
6. If the fish is still eating, offer medicated food for internal bacterial infections, as it is often more effective than water treatment alone.
7. Avoid overfeeding and remove any uneaten food promptly to maintain excellent water quality.

Please share the following details so I can guide you more accurately:

• A clear photo of the fish (side view and top view)
• How long has the swelling been present?
• Are the scales sticking out like a pinecone?
• Is the fish still eating?
• Tank size and current water parameters (ammonia, nitrite, nitrate, pH)

Dropsy can be difficult to cure if it is in an advanced stage, but prompt treatment and maintaining excellent water quality can significantly improve the chances of recovery.
Regards,
Yashwant Naik