this is the method I have used
Buy pure clove oil. You can get it at a health food store. Put the fish in a medium sized mixing bowl in his own water from his tank. In a small jar or something with a lid, such as a jam jar, mix the clove oil with tank water. Put the lid on and shake it like crazy over and over until the liquid in it is white. Then pour a little into the mixing bowl with the fish. Swirl it with your hand. The fish might fight it just a little bit and then slow down. Then pour a little more in and swirl again. He should just go to sleep and appear dead. If he doesn't, try a little more of the clove solution, always shaking very well before an addition to the bowl. When he goes to sleep, leave him in the solution for a good 30 minutes.
Pain free death. Very humane. We should all go so easily.
as for the ammount of oil, 1 to 10 of tank water.