I just put one of my Betta's that had severe fin rot out of his misery today. Put osme water in a little plastic container in the freezer, waited till it had iced over and broke the ice. Netted him from the tank and dropped him in the water. He went motionless, went nuts, then stopped. He's in the freezer at the moment. Much nicer than dropping them in boiling water. (The above quote was mine )