Clove oil has to mixed with water before using it. The usual way is to put the fish in a tub of tankwater. Put 25 to 35 drops clove oil in a screw top bottle and add some cold water. Shake the bottle till the oil has emulsified, then pour into the tub with the fish. The fish will get upset for a few seconds, then drift to the bottom. The danger is that if you remove it too soon, it will just be unconscious rather than dead. Wait at least two hours before disposing of the body.
As for the amount of clove oil, 25 drops are for a small fish in a small tub of water; 35 is for a big fish in a big tub as the larger water volume would dilute 25 drops too much.