Clove oil, or cut its head off, it it's a very small fish crush it quickly with a FLAT brick/rock on a FLAT surface, it's instant, I had to do this to a guppy once, (I couldn't find clove oil) and it was instant. NEVER flush a fish! putting them in hot water(esecially wide bodied fish) causes them alot of pain before they die, which could take a while. Vodka/alcohol is also a terrible way to go, my friend tried it(I was there) with a tetra, it thrashed around and tried to jump out for about 15 secs, then we just cut its head off. It was obviously in severe pain.
I'm sorry about the goldy