In my office we have a gold fish that is alive but floating upside down in one corner but swimming unless it is 'tickled' from the outside of the tank when it swims across the tank in a normal fashion, on monday it was corkscrew swimming.
The office is split between humance euthenasia and giving him tonics etc etc.
How can you tell if the fish is suffering / in pain? How to decide when to euthanase if at all? If killing him is decided upon what is the most humane method?
It might be diet related, that's the easiest thing to rule out. If you feed it a cooked pea with the skin taken off then that will get things moving if it's constipated.
It could be environment related. This is a very common one because people are very rarely told how big goldfish get and how much space they need. How big is the tank? Unless it's 30 gallons (yes, gallons) plus then it's too small. This could be causing water quality issues which in turn can cause all sorts of illnesses that can interfere with the swim bladder. If you get the water tested then you'll have an idea, or if you tell me the cleaning regime, tank size and number of fish then I can take an educated guess.
The other possible cause is genetics- fancy goldfish are bred to be a certain shape and their health can suffer because of this. Think of the health problems with some pedigree dogs but worse and that's fancy goldfish.
Euthanising a fish is fairly simple, you need a bottle of clove oil. It'll be with the toothpaste and whatnot in Boots. Have a go with the pea and the water quality first to see if you can improve things and if it looks like curtains for the poor fish, I'll talk you through euthanising. Please do not let anyone flush it while it's still alive, it's a horrible end for them.