As a few have said the best method for euthanasia is using clove oil. As far as decapitation goes you need to stun the fish first as nerve tissue is tolerant of anoxia, and the brain may continue to function for some time after decapitation. Now on to the OP's question as far as freezing fish. Many fish are able to cope with very low temperatures and just because a fish cannot respond to a stimulus when hypothermic does not guarantee that it cannot perceive it. I have seen it mentioned that you can place a fish in a small amount of water and add several of those CO2 tablets or alka seltzer tabs(or if you have a CO2 system) put in large quantities of CO2 but to me that seems would not be the fastest way for euthanasia. I would still go with the clove oil method as it seems to be the method that is most widely accepted as fast and humane as it can be.
All matter is merely energy condensed to a slow vibration, we are all one consciousness experimenting itself subjectively, there is no such thing as death life is only a dream which is an imagination of ourselves. Bill Hicks
ua hua's 90 gal. high tech
ua hua's 5,000 gal. water garden