I was interested in the methods people used so just had a search around the web.
The freezing method is widely mentioned to and referred to as humane, but from what has been previously stated in this thread I would tend to doubt that. However I found this article on a site called www.koivet.com, it is aimed more at Koi keepers, but it does have some relevance (I have edited for length):
Euthanasia in fish
Ideally, hypothermia is the recommended way to dispatch a fish.
Place the fish in a modest amount of water so that it may recline upright and comfortably. It should not spend its last moments frantic for water. Place the bowl (or bag) in the freezer and close the door, providing darkness. As the poikilotherm (cold blooded animal) loses heat, the enzymes that sustain consciousness are inactivated and they lose sensation and finally all systems eventually arrest, painlessly and completely.
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Freezing really large fish WOULD indeed be cruel from a logistical standpoint because of the physical realities of putting a twelve pound fish in the freezer. Good point. I didn't think of that. My advice on freezer euthanasia refers to the average six to eight inch household goldfish.
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Another person mentioned, correctly that their fish are not unconscious at thirty something degrees during winter. The premise becomes that the fish in the freezer is also conscious at thirty something degrees while it dies. This would be true if the fish came from an icey pond and you tried to freeze it to death. If the fish were from your living room, it would be shut down with the first twenty degrees drop in water temperature. The reason for this is "iso-enzymes" - It has to do with how poikilotherms, (cold blooded critters) survive in hot, then frigid environments.
In warm weather, cold blooded fish use warm-water enzymes. As it gets cold, they produce and use more and more "iso-enzymes" which function in ice water. This is gradual and natural.
A rapid rise in temperature destabilizes the cold-water iso enzymes structure and the fish dies. The opposite is true of the warm weather poikilotherm enzyme system.
Decapitating or hitting the head hard seems to get several votes and does sound better. Difficult for a small guppy or tetra though. Another point was raised that if decapitating you need (1) a SHARP knife, (2) a good aim and (3) no squeamishness - if you use this method it needs to be one chop and done. I would hate to imagine what it would be like it you only half did the job
Chemical, etc. methods
Alka-Seltzer - opinions differ with some saying it is quick, others saying their fish seemed to be suffering. Basically it seems to work by suffocating the fish with the CO2 from the Alka Seltzer.
Vodka - opinions differ again. Methods range from neat grain alcohol to a 1 in 5 solution of vodka or tequila. One site even felt it had to remind people to dispose of the alcolhol after use rather than drink it....
Bleach - Found this on one board:
"If you drop the fish into a glass or plastic container with undiluted chlorine bleach, death will be instantaneous. I once got rid of some guppies this way and the fish did not suffer. As an added bonus, I left the fish in the container for a few minutes and the sodium hypochlorite disolved most of the fish making disposal easier." Not at all sure about this one....
MS-222, a fish anesthetic - Quote from www.aquariumsite.org:
"It can be purchased from chemical supply companies as MS-222, tricaine Methanesulfonate or Ethyl 3-aminobenzoate, methanesulfonic acid salt. Immerse the fish in a container of 350 ppm MS-222 (350 mg MS-222 per liter of water) for 10 minutes. This is very humane and is non-traumatic for both the fish and owner."
Flushing down the toilet - this is a form of torture as the fish ends up in a septic tank or sewer where it can potentially survive for hours if not days bathed in nasty chemicals and sewage before finally dying.
The last one I could find (on a quick trawl) was a suggestion that a local college or university might be interested in taking the fish for biology/anatomy classes. I am not sure about this as would you want one of your fish in the hands of the average student.....
I must stress that I am not an expert - the one time I had to euthanize I used the freezing method as this was recommended by someone. I have gathered this information from a lot of sources on line and I think it should all be treated as OPINIONS rather than out and out fact - with the possible exception of the MS-222 method.
Hope this is of interest, Eddie