My aquarium fish is sick and I don't know the least painful way of putting it down. How should I kill it so that it doesn't suffer?
Depending on the size and temperature limitations of the species, there are several ways approved by the American Veterinary Medical Association.
One-step process of immersions (intentional overdose via immersion) in easily obtained (for hobbyist) solution includes:
It is important to note a concentration of 95% ethanol is humane and CO2 saturated water should be saturated from pure sources, such as CO2 canisters.
My preferred method is clove oil as it is cheap and painless. It acts as an anesthetic to the fish. I remove the fish from the tank and place it in an easily cleaned or thrown out container of tank water. I take another container (usually a pill bottle) and put 3–5 drops (for fish up to 4 inches in length) of clove oil along with tank water and shake until the water is milky. Then I add this slowly (while stirring) to the container the fish is in. The fish will slowly go to sleep and will flip over and float belly-side up. This is just the fish being knocked out. I then mix another concentrated bottle of clove oil with water and add that to the container which causes the fish to die. Depending on the size of the fish, adjust the concentration. I leave the fish for about 10 minutes and then check for gill movement. I've had pretty decent success with this method with a very low percentage of fish showing any signs of distress.
I keep clove oil on hand. If you don't have access to these methods, there are physical methods described in the AVMA Euthanasia Guide.