I've personally "euthanized" a lot of fish, but that's because my father and I used to catch fresh fish for dinner, bring 'em home, fillet 'em, and cook 'em. I couldn't tell you how many fish I've had to do this to, but I can say with 100% certainty that severing the spinal cord right behind the skull (aka. cutting the fish's head off right behind the gills) is the quickest and most efficient method of disbatching them. My father and I would stick the live fish in a cooler half-full of ice before we get in the car to take them home, and they'd still be moving around a little bit once we got them home, (half hour drive) so I highly suspect that the freezer method is very quick. It very well might numb the fish to a point of not feeling pain any more, but I've personally never frozen to death, (and nor has anyone posting on this web site) so I wouldn't know. I've heard of the baking soda method and seen it performed on a terminally ill oscar (it was quite obviously in a lot of pain... it had severe hole in the head, along with severe fin and body rot... and to answer the question as to why I saw it done, I work for petsmart, and that was what the district called for at the time) and it was thrashing around for at least 15 minutes before it finally gave up. I must say... I would think that drowning is one of the most painful experiences a person could go through... and baking soda is the key to drowning a fish. I know its messy, and difficult for a lot of people to overcome the thought of, but decapitation is in my honest opinion the quickest and least painful method of euthanasia for a fish. (although I am intrigued by the idea of clove oil...)