MAMAFISH68, yes, I have used the 'freezing' method to kill fish in the past (before I knew it was inhumane). I had actualy chosen, before then, to use very cold water (almost freezing - often with a layer of ice on top) to 'shock' the fish to death. After it didn't work a few times, I started asking around for other ways of doing it and was told of this way and I used it - it worked just fine. Later I found out just what processes are involved (ie: the crystals that form inside the fish' body and how slow the death is). It's true that the fish eventualy die and it's nicely outside of your view so you can't see the fish dying which many people find most convenient - but ice crystals also begin forming inside their bodies, damaging organs and literally freezing their blood. A lot of fish can also survive for quite a while as temperature drops. This is extremely stressful as anyone who owns fish should know - fluctuations in temp. typicaly end up resulting in disease due to stress. Also, you are incorrect in assuming that the fish becomes 'dormant'. This is true of many coldwater species (eg: goldfish) which have adapted to lower their metabolism as temp. drops so that they pretty much 'sleep' during cold times. However, this isn't quite the case with most tropicals and they don't in fact die until their bodies are beginning to freeze (or until they suffocate in some cases - without filtration, in limmited water or in a bag like some people do this...). Besides which you need to consider that the temperature drop in a freezer is far more rapid than is likely to ever occur in the wild which means that the fish doesn't go into a dormant state before it begins freezing. Another thing is that, occasionaly, a fish will be placed in the freezer but removed before it's actualy dead (though it's likely to be severely damaged internaly already). It may easily revive and end up suffering for a much longer time before it dies.
The whole point of 'euthanasia' is to provide a painless and quick, as stress-free as possible death. Euthanasia literally means 'easy death'. Besides the issues I mentioned that need consideration, it seems pointless to me to use such a time-consuming and inhumane method of killing fish when there are far better options available. Clove oil is the best and it's both quick and easy and it involves far less stress on the fish' part and no pain. All you need to do is put the fish in a bowl of tank water and add a few drops of the oil. the fish drifts off almost immediately. The more you add, the quicker it dies. Then you can either leave it in the bowl an extra while to ensure it's dead or destroy its brain to remove all chance of revival. Clove oil is readily available at your average local pharmacist (usualy for use to relieve toothache) and doesn't cost much at all either.
Actualy, if you have something very specific against using clove oil, you can just crush the fish' head (or remove the head altogether with a sharp knife). As long as the fish isn't kept out of water too long and the fish isn't something that's likely to 'flap' away or that's too big to kill instantly, this is also humane because of the speed of it. Many people, however, opt for the clove oil method because it's less unsightly and doesn't require a strong stomach.