I just use oil of cloves(available at any health food store), take some aquarium water, add some oil of cloves in(like 10 drops), shake till uts milky white. ADd fish, wait till it doesn't move.
The oil of cloves is a light anesthetic, so it guaranteed to work for small tropical fish.
20G heavily planted tank with Eco Complete
Pressurized CO2, Eheim Ecco 2236, 1 55W and 1 36W bulb