Ive read these posts before about euthanasia to fish. Its a sad subject but I suppose its bound to come up once in a while.
IMO I don't know if there is a humane way to kill a fish, after all who knows what they feel. I mean would you like someone to chop your head off because your ill.
I suppose they only 'Humane' (ie a way that humans approve of) way that we can relate to in terms of killing is the electric chair. So connect him to your car battery an voila! Only joking
dont take it seriously.
If on of my fish had a proble i would probably smash it over the head. Its the best way IMO. Or better still as long as it wasn't deseised or anything, just nipping fin damage i would try and feed it to a bigger fish, Just as nature intended!