I put them in a cup with just enough water to cover them, I use party punch cups.
Wait for them to take a breath then pour in isopropyl alcohol "rubbing alcohol" or if you're maudlin use expensive 190° vodka.
When you get the alcohol percentage past 20% they should pass, be sure to give them some time in the cup this way as you don't want them coming back.
It's almost painless and a hell of a lot faster than having their labyrinth gland frozen and suffocating to death from brain freeze.
It's also sterile. I do this then put the cup out beside the fern and bury them the next day.
A half liter of 50% will do five or six fish and costs less than a buck.
Or you can use hot water over 120°. Its gruesome but fast. Remember to take bye-bye pictures before hand if you use hot water.
I sometimes take pictures after alcohol as part of post mortem.
Last edited by Thunderloon; 03-07-2011 at 01:38 PM.
Reason: hunger was making me nuts