Hi Joan-e first off we need to know your tanks water perameters Ammonia Nitrite, Nitrate, Ph to give us an idea if there is anything going wrong in your pond, next off can you firstly tell us what type of fish this is i;e koi, goldfish or other?
Looking at the fish can you see if any of the scales are pineconing ? Dropsy is a difficult thing to treat as it may be due to quite a few factors , for one there are differing types of dropsy .
Firstly there is internal bacterial infection next the viral and lastly a parasite that attacks a fishes kidneys our gues yours has an internal bacterial infection it being the most common .
It sounds as this poor fish is now to far gone for any sort of help but you could try putting the fish in an areated tank and adding salt to the water at 5-6kg per 1,000 litres of water which should help reduce osmeticpressure on the fish also raising the temperature of the water at a degree a day up to and above 25c .
Will it help I dont know personally weve never heard of a fish that is belly up and swollen like it is coming back from the edge.like this in all our 27 years of koi keeping
Dave