I've done the clove oil without the alcohol and it worked fine for me.
I put my angel in a small bucket with aquarium water, added a few drops, she calmed right down and then she slowly went to sleep, added a few more and she was gone not long after.
It was slow but it appeared to be relatively humane.
4 ft 60g FW: 2 angels, 4 blind cave, 1 whiteskirt, 4 gold widows, 6 serpaes, 1 neonhead apisto, 4 clown loaches, 4 albino cory, 2 bronze corys & 3 BN plecos.
23g FW: 1 neonhead apisto, 1 yoyo, 2 albino BN plecos & 4 pepper cory
5.5g: Crowntail Betta, 6 black Khuli's & RCS
5 g: Crowntail Betta, 1 albino cory (Quasi), 4 baby khuli's & RCS
7 g: Dragontail Betta & 2 albino cory
3ft Axi tank: 1 black, 1 gold - 1 redcap oranda & 2 albino cory