Man, it sucks that you had to put it down; I know how hard that is. No one wants one of their "babies" to suffer, and we all do what we think will minimize that suffering - That's why we euthanize in the first place. Brutal as it may seem, I've used a metal mallet in the past; harder on me to do it that way, but the quickest way I could think of to end the suffering. The smacking on the pavement method would bother me, because I couldn't control where the blow is falling and it might not be immediately fatal. Also not so sure about the ax method since it doesn't shut down the brain. Not comfortable with freezing - I keep thinking that it takes a long time, carbon dioxide means you're suffocating the fish, clove oil - don't know what to think. Mallet seems the most efficient way to me, but it is SO hard to make your arm fall! Anyway, sorry for your loss...
