when a fish is as ill as yours it is best to put it down,it is not humane to wait and let the fish suffer any longer.
the entire body of your fish is swollen it do probably have droopsy(ascites) and this is an illness where the chances for survival is slim,droopsy is an illness caused by virus and is common in the carp family.
one way to eutanize fish is to use clove oil it is painless and quick for the fish,the reason for using this method and not using the icecold water method is goldfish and many other types of the carp family have no problem in surviving in water down to the freezing point of water.as long as the water is not frozen the fish will survive(they do under the ice in my garden pond).
What's the least painful way to euthanize a fish?
a little about droopsy(ascites)the illness in fish is often caused by virus and or bacteria the result is heart and liver faliure and this leads to the fluid accumulating in the tissue of the fish,when we fishkeepers see the signs it is often too late to save the fish.