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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?

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up vote 16 down vote accepted

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:

  • Eugenol (clove oil)
  • Ethanol
  • Carbon dioxide saturated water. For most fish

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.

  • DO NOT flush the fish while alive as the chlorine in the tap water will burn the fish's gills or they may not die.
  • DO NOT leave the fish out of water as this is not humane.
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There's a tale about a frog being warmed inside a pan with water and not feeling anything until it is dead. May that work with fish? –  sharptooth Dec 23 '13 at 9:26
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The AVMA advises against slowly freezing or boiling. If need be, the transition from normal temperature to the extreme should be as quick as possible. –  Raystafarian Dec 23 '13 at 10:43
    
Oh, but keep in mind, freezing will only work for tropical fish (not koi, goldfish, or other fish that can survive at low temperatures.) –  Raystafarian Dec 23 '13 at 12:04
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95% ethanol is probably more painful than putting it through the blender .... –  Chris Dec 24 '13 at 15:39
    
@Chris and probably slower too... –  woliveirajr Jan 8 at 12:00

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