I usually do it with alcohol. If the concentration is high enough (pretty VODKA much straight out of the bottle) it will only take a few seconds. The good thing about this too is that I don't have to place a nasty sick fish into my freezer with all the food to completely freeze him.
Previously: 2x 10gal tank, 45gal aquascaped, 70gal aquascaped
Currently: 16gal FW, 40W CF, ADA Aquasoil Amazonia II/Tahitan Moon Sand, PH 6.5, kH2, gH 4