Perhaps we anthropomorphize our fishes too much. Check this out:
http://uwadmnweb.uwyo.edu/Zoology/faculty/Rose/pain.pdf
here's a brief, oversimplified summary for those who don't want to spend an hour reading about fish pain:
Turns out fish lack the brain centers we use for experiencing pain and fear. They have a glandular responce to bad stuff that makes 'em look scared but they can't 'feel' it the way we do. Bad stuff is still bad for fish because they can't controll their responce so they can bash into rocks and hurt themselves or hide untill they starve etc. --> The ethical aquarist will still try to take good care of his/her fish and avaid 'scaring' them because it can hurt them to be 'scared.'
--> euthaniasia can be whatever makes you comfortable.
That said, here is a list of IACUC (institutional animal use and care commission) approved euthanasia protocals. Interesting that most have been arrived at independantly by home aquarists.
Fish
Tricaine methane sulfonate (MS222)
Benzocaine
Barbiturates
Inhalant anesthetics
CO2
2-phenoxyethanol
Conditionally acceptable - stunning followed by decapitation/pithing; decapitation and pithing
Decapitation of fish, amphibians and reptiles should be followed by pithing. Use as a sole means of euthanasia in any species requires scientific justification and IACUC approval.
Under very specialized circumstances, stunning, rapid freezing or air embolism (under anesthesia) may be allowed in small species if research needs make it necessary and there are no available alternatives.
taken from:
http://www.ora.ucr.edu/vet/Primer/bi...nasia.htm#FISH