A search through the NG for "clove oil" should bring up more
information, but clove oil is sold as an oral anesthetic, and
overdosing your fish with it is considered a relatively humane way to
euthanize them. It's also used in small amouts to temporarily
anesthetize them to take care of wounds.

from an older post:
"The proper dose is 1 cc of eugenol (clove oil) + 9 ccs vodka to one
gallon
of water. Or for larger fish 2cc of eugenol (clove oil) to 18cc of
vodka to
2 gallons of water. Use the same ratio of 1cc eugenol/9cc vodka for
every
gallon of water you use. Mix the clove oil and vodka together before
adding
to water. After you add the mixture to the water, place the fish in the
solution. LEAVE THE FISH IN THE SOLUTION FOR APPROX 10 MINUTES. The
results are very quick and painless."

hth.