stamp on it
How the hell would that be instant? If you were dropped in boiling water, it certainly wouldn't be an instant death.Either drop it into boiling water or throw it hard against a hard surface, 2nd option is a bit messy so the first one is instant
Ive owned tropical fish for years and the most humane way to kill a fish is to put it into a small container of freezing water and place it in the freezer.
Fishy will go to sleep.