I have used the clove oil method and, done properly, it's
very quick; much quicker than the freezer method would be, which
is frowned upon by vets nowadays and they
do think it's probably painful.
The lethal dose is 30 drops per litre.
What you do is take a litre of water out of the tank, into a container that you won't want to use for anything else (the smell of clove oil really lingers) and put your fish in it.
Remove a small amount of the water into a screw top jar, add your 30 drops of clove oil and shake until it becomes a white emulsion. Then just pour that slowly into the container. The fish will be unconscious in a few seconds, and be dead, I'm sure, in a minute or two.
If you use enough clove oil there;s no need for vodka or anything like that (I'd drink the vodka myself; you need a stiff drink afterwards; it's not a nice thing to have to do
)