With a small fish I worry that I won't hit it in the right place and it won't die immediately.
It's hard to tell what is happening to the fish with clove oil, I believe their heart rate gets slower and slower till you realise their gills have stopped moving and they seem dead. It takes about 5 mins though. It's anaesthesia overdose and it looks to be a peaceful way to go at least.
I put the fish in a little container and then drop the oil onto the water, then gently stir it about a bit. They flap their gills a little and maybe do a zip around the container but then they stay still. I leave them in there for an hour or so longer just to make sure they are definitely dead.
Freezing them is now considered inhumane so I would rather not do that.