Agreed on the clove oil. If you want it to work even faster, mix it with some alcohol, it'll mix into the water faster and hit the fish quicker. Having said that, I've always frozen my sick ones. If they're a little too big to die quickly enough (i.e. without thrashing), I usually knock them on the head, or stab them in the brain case which kills them pretty quickly. I'd only reccomend the latter method if you had a steady hand and a good knowledge of fish anatomy, however. Fish have small brain cases in a big head, if you don't know what you're doing, you'll just put holes in their head and they won't necessarily die any faster.