I wanted to compile several different contributions into one thread, and make it a sticky. You have two options to use for euthanizing a fish: MS-222 or Pure Clove Oil. For MS-222 you can find more info and euthanasia dosage here: http://www.advancedaquarist.com/2010/11/fish You can buy MS-222 from here: http://www.wchemical.com/tricaine-s-ms-222.html Below is video documentation from @Steve Jones showing euthanization of a sick wrasse using MS-222. Below are instructions from @Lionfish Lair on using pure Clove Oil (available at many drug stores) for euthanization: • Put 3 drops with a half pint of water and shake very well, so the oil and water make a fusion - otherwise the oil will just float on top of the water and for the euthanasia to work the fish has to get the oil into its system. • Add the mixture to the water that the fish is in (1 gallon of water should be more than enough) and stir it around slowly with your hand. The fish should become lethargic and sleepy. When the fish goes "belly up" it is asleep - not dead. • Then add 3 more drops of clove oil. Add another mixture of 2 to 3 drops of oil in water. • The fish feels nothing, it is very peaceful and humane. • Don't make the mistake of thinking that if you initially put more in it will act quicker - it will only freak the fish out - it has to be done gradually so the fish doesn't notice the change in the water. • The whole procedure should take anywhere from 30 minutes to an hour. You will know when the fish is completely dead because there will be absolutely no movement of the gills. If after an hour the fishes gills are still moving you could add a couple more drops of clove oil. Sometimes it does depend on the size of the fish to how many drops of clove oil are needed. • Properly dispose of the deceased fish and wash treatment container thoroughly with soap and water. And this video from @Duke4Life shows how to use Clove Oil to euthanize an injured wrasse. I want to thank one of the most knowledgeable ladies I know for the clove oil write-up; and both of these fine gentleman for their very informative videos, although I realize they were difficult to make.