Before you start a medication or treatment of any sorts go 24 hours without feeding the fish to rule out over eating. If by the next day the fish is still inflated then explore the next options.
What is Boat/Dropsy?
It is a bacterial infection of the kidneys, causing fluid to build up or renal failure. From the fluid build up it causes the fish to bloat up and scales to protrude. It appears to affect weakened fish and fish in unkempt conditions.
For fish that are salt tolerant:
*Remove carbon/resin from filter
*Do a 30%-50% water change
*Add salt for your tank size
*Treat with Metronidazole (Flagyl)
*Leave for 3 days and do not feed
*After 3 days, do another large water change
*Add full dose of Metonidazole
*Wait another 3 days, Bloat should be decreasing in appearance. The water may take on an ammonia smell as the fish should now be producing urea.
*3rd day (6th day)do another water change. Replace salt and do another full dose of Metronidazole. If the fish looks to be on the road to recovery offer a small feeding each day.
*Another water change on the 3rd day (9th day). No meds but replace salt. Fish should be better now.
For fish that can't handle salt or if you choose not to use salt:
Add an antibiotic to the food. With flake food, use about 1% of antibiotic and carefully mix it in. If you keep the fish hungry they should eagerly eat the mixture before the antibiotic dissipates. A good antibiotic is chloromycetin (chloramphenicol). Or use tetracycline, erythromycin or minocycline. If you feed your fish frozen foods or chopped foods, try to use the same ratio with mixing. As a last resort add at most 10 mg per liter of water. And as always keep up on the water changes.