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.
Freezer - This has long been a favourite method and is widely recommended even now. The fish is said to drift off to sleep as the temp lowers. In actual fact it is one of the cruelest and slowest methods of all and it will involve a great deal of suffering.
Fish torturer!