If you are asking for the best way to euthanize them it is Finquel (MS-222). It is accepted as the most humane method in the trade. However, I understand that is a chemical not normally available to most people. Clove oil is a good alternative; it can be found at your local pharmacy. Here is an excerpt from this website, Euthanasia
, about how to euthanize with clove oil:
Add 12 drops of Oil of Clove per 1 gallon (3.84 liters) of dechlorinated water. Oil of clove is very hard to mix so you may want to emulsify the clove oil by placing it in a smaller amount of water in a glass jar and shake well. Place the fish in a container large enough for it to move around freely along with an airstone on the highest setting and add the clove oil solution. The airstone will help keep the clove oil in solution. Once the fish turns on its side, watch the opercular movements (respiration/gill movements). The fish should be removed from the solution after at least ten minutes have passed since the last observed opercular movements (respiration/gill movements).
If you do euthanize your fish, please be kind to them. While the debate is still out in the scientific world, many of us believe that fish do feel pain and fear.