Here's the way I euthanize fish, though, and since I've killed many
thousands, I can say it's the cleanest, least painful way to do it,
for both me and the fish. It's an anesthetic used for frogs and fish
that goes by various names: ms222, MESAB, 3-aminobenzoic
acid ethyl ester, tricaine methanesulfonate, or, as most of the
pet and aquaculture supply houses call it, Finquel. For routine
anesthesia, I use a 0.2% solution of the stuff—let a fish swim in
it for a few minutes, they lose consciousness, you can do various
surgeries on them, and then put them in clean fresh water,
and a few minutes later, they're awake and swimming around
again. If I need to euthanize them, I use a 0.4% solution (or more
crudely, I use my 0.2% stock and sprinkle a few extra crystals of
the ms222 powder in the beaker), put the fish in it, they fall
asleep…and after 3-5 minutes, their heart stops. It will kill them
at lower doses, but simply takes longer.
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