This might sound cruel but for euthanizing an animal I find the fast and direct approach to be the best. I would scoop the fish out of the tank, take it outside, and throw it as hard as you can onto a cement or other hard surface. I have done this on a few occasions and the fish are not gasping, twitching, or moving in anyways afterwards. I have heard clove oil works, but I haven't seen a study that states they are anesthetized, so I worry that the clove oil might be working like a muscle relaxant, which would mean the fish cannot move or show outward signs of distress but in reality could be in a lot of pain and panicking.
I do have a rule when euthanizing fish. If the fish is very active in evading capture it is not ready to die yet, I leave it for another day in this case.
It requires a bit of strength to do this right, if you don't have the strength then consider using a hard surface and a hammer on the head of the fish.
This is only my opinion other's might think differently.