So sorry, I am a total newbie here, but when I saw this thread I had to step in, even though it may be too late. I have been in the aquarium hobby for 20+ years now, and know all about euthanizing fish. I actually researched this topic and wrote an article on it a few years ago.
DO NOT:
use alcohol: it burns their skin and gills, very cruel
boil: you are cooking them alive, cruel
freeze slowly: also painful as ice crystals form in the cells
etc etc......
The only truely humane method, for small to medium sized fish is the ice water method. I do not even turst clove oil, as I did try this once, and my fish acted like he was just dropped into acid, he freaked out and it took a long time for him to die. Another thing about clove oil, it is supposed to be an anesthetic, so if one does not leave them in it long enoguh, and then they dump them in the thrash or the toilet, they may start to breath again. It is recommended if you use this to keep them in this solution for a few hours to insure death has occured, or chop their head off after they appear dead.
Ice water: Put a small container, big enough for th fish of course, into the freezer for awhile..until a layer of thin ice has formed on the surface. Break this...catch your fish, and submerge completely and quickly. This is instantenious, as it is the SHOCK that kills him. His heart stops..he does not even feel the cold water, he dies within a sec. or two..most times I dont even see a twitch form the fish I have done this to, they just spasm into a death position, with their back curced like a "u". No pain, I promise.
For larger fish, like big cichlids and goldies, the guillitine style is the best...chop the head off. Hold the fish in a towel so you can get a good grip, leaving the head exposed. Slice with a sharp knife directly behind the gills. A steady hand and good nerves is required.
Here is a link to a more detailed essay about the topic, which I wrote:
http://bcaquaria.com/forum/viewtopic.php?t=1867
Also sorry for jumping in like t...will be happy.
Thanks for listening.
Mindy