Then again cats aren't fish, can't really compare apples with bananas. But if you had a fish the size of a cat, well that could be a different story
Humans die quickly similarly, the brain thrown against the skull does the lethal damage.
Whenever we'd catch sprats for the cats at home, we'd kill them instantly by throwing them on the jetti with a lot of force, much better than most people that let them sit slowly dying in a bucket because they couldn't bring themselves to do it, that is more cruel undoubtedly.
I'm sure someone proved the whole 'blood crystalising' when freezing idea totally wrong, I think it's impossible for that to happen.
Clove oil sounds good, anaesthetic is always good