I use the freezer with the fish in some tankwater. They just slow down and then stop. I don't think it's inhumane for non mammals as it's the method used for smaller reptiles and invertebrates that are a biosecurity risk. The ideal method would be by anesthesia and oxygen displacement.
The smacking of the fish on concrete is a bit gruesome for me, and if it doesn't work first hit it's cruel, and what says you're not just paralysing the fish?