with larger fish, such as Cod or Salmon a good crack between the eyes with a "Priest" a fishermans name for a cosh usually does the job. I've also seen the freezer recommended for small tank and pond fish
I have a tropical fish tank, I do love my fish but when i know there is no hope i normally have kitchen roll at the ready hook the fish out asap and fold it in the roll and give it a quick smack with the palm of my hand, I know it is not everyones cup of tea but it is very quick and thefish does not know what has happened But i would never freeze it poor thing would just suffer.
It is the minimum requirement to stun crayfish before chucking them in boiling water, although many don't.
Fish can stand very cold water for a while and if they come from the colder seas they have a sort of anti-freeze in their blood.
The sudden death solution is much better and painless.