Clove oil is also a fish anaesthetic and can be used for putting a fish to sleep, if you feel it's suffering.
You put the fish in a container, with a measured amount of water. Take a little of that water and put it in a jar (you need to be able to mix the clove oil into the water). Add the clove oil to the jar; you need 25 drops per litre of water. So, if your container (with the fish in) holds two litres, you'll need 50 drops of clove oil. Shake up the jar until the oil is mixed in, and not sitting on top, then add that to the fish's container. The fish will become unconscious very quickly and die soon after.
It's a reliable, humane method, recommended by vets.