Do you know the water parameters?
Do a few 50% pwc
's one after the other. Get the water in top shape.
Do you have aquarium salt or kosher salt or sea salt with no additives?
Do you have Epsom salt? No additives, oils fragrance, etc.?
What kind of fish is it?
Do you have an extra filter?
Can you buy a splitter for the airline tube - a T or Y connector to use you present air pump.
Walmart might also have it. Check under the sofa cushions, or the vehicle seat for some change, (in closet for handbag, backpack or wallet, etc.
Dollar store usually has $1. sea salt or kosher salt (make sure no additives.).
Generally when the fish has dropsy the internal organ damage is already at a high level. (reason why the fish is retaining/ not processing the fluids) Not maintaining osmotic regulation.
First thing is get the water perfect. The reason recommending several pwc
's instead of one huge one is the gentle change of water quality which is less shocking to the ill fish's system.