Instead of culling a SBD goldfish, why not give it a last shot by removing some air from the swim bladder? I saw an interesting article on how to remove air from the swim bladder. Check this out.
http://www.clevelandaquariumsociety....er_surgery.htm
http://www.mnr.gov.on.ca/228130.pdf
To locate the swim bladder, check this out. Warning: don't see if you have a weak heart.
http://thegab.org/Articles/GoldfishInternalAnatomy.html