This is a very hard thing to do. I sympathize.
I have tried clove oil to death, and without going into details, I am uncertain this is the best method. I have heard in paralyzed the fish, and they do stop breathing, but it seems rather slow to me, ime.
I have now turned to dropping the sick fish in very ice water, with cubes, and they drop instantly. Make sure not to let the water warm, so I then put them in the freezer. This has worked the best for me, I have seen no suffering.