From Pharyngula comes advice on the best -- for the fish -- way to end a life with dignity and as little pain as possible. Hint: ms222. I don't think they sell it at Walmart.
We get to the lethal injection concept only after some really nasty (and perhaps flawed) euthanasia alternatives are discussed:
Less nasty techniques are the freezer and alcohol strategies. I don't think putting a fish in a freezer is humane: they don't seem to react strongly to slowly freezing to death, but then they can't - their metabolism is shutting down. Fish tend to be very sensitive to cold, though, and seek out optimal temperatures and avoid the cold, and can respond to changes of a few degrees with shock, so I have my doubts that this is a good way for them to go. Putting them in water with a few percent alcohol might be OK; they do get drunk, pass out, and die, just like people can.OH! I thought that was how we were supposed to euthanize Aunt Hilda!
But fear not, you too can be the Dr. Kevorkian of fish:
I get my stuff from Sigma, catalog number a-5040, for those of you who can purchase through academic suppliers. Otherwise, here are a few commercial places that will sell it to you: Doctors Foster & Smith, PondRX, and Argent Labs. It's about $15-20 for a 5 gram bottle, which sounds expensive, but a little goes a very long ways. I bought a 25 gram bottle 8 years ago, and I've still got lots left -- and I euthanize fish far more often than your usual pet fish owner.
You know. Some questions are just better left unasked.