Whenever I have these problems, I just dip the head of the fish into boiling water.
Dies instantly without even a twitch.
Then I soak it in the boiling water long enough to make sure any contaminants in it are gone, and then toss it to my Arowana--who greedily gulps it down without a second thought.
Humane and nothing goes to waste.
Not everyone has a greedy Arowana though.