Found this on a guppy board I visit, although, as "uncle Scott" says, it has been taken from other sources...I thought it was interesting, given the original topic...
(Uncle Scott says
"I found the following (taken from a note on the rainbowfish mailing list)useful in thinking about the issue of aquarium fish euthanasia. As more data becomes available, the scientific community changes it's thinking on these issues too. I would also guess not everyone would agree with it. Still it is worthy of your consideration:
The Australia and New Zealand Council for the Care of Animals in Research and Teaching have a publication titled "Euthanasia of Animals Used for Scientific Purposes", has three categories of assessment of euthanasia techniques for fish. These are recommended, acceptable with reservations and not acceptable.
Recommended:
Halothane, MS-222, benzocaine, eugenol, clove oil
Acceptable with reservations:
Injection with Sodium pentobarbitone (Stressful because of need to remove from water)
Stunning and brain destruction, cervical dislocation, decapitation in stunned or anaesthetized fish
Not acceptable:
Carbon dioxide, Cervical dislocation in large fish, decapitation alone, removal from water, hypothermia/ freezing. (The reason they say the use of cold shock is unacceptable is the length of time it takes the fish to become unconscious."