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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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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 / 10 cm 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 upside down. 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
    Commented Dec 23, 2013 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
    Commented Dec 23, 2013 at 10:43
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    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
    Commented Dec 23, 2013 at 12:04
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    95% ethanol is probably more painful than putting it through the blender ....
    – Chris
    Commented Dec 24, 2013 at 15:39
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    Whew. The most humane way to kill me would be more like chopping my head of or opening blood vessels, not to flush me with drugs and chemicals.
    – phresnel
    Commented Dec 3, 2015 at 11:20
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Sorry for posting on such an old topic, but I cannot agree with the accepted solution - so, for the future:

Causing the fish suffocation or chemical burns is not humane, unless for you it means "clean" rather than "painless" - be sure you understand your own motivations.

Shots and blunt force trauma to the head are also approved in the AVMA Euthanasia Guide for pretty much anything. Besides immediately disabling the nervous system, it leaves no time for feeling distress from unfamiliar circumstances.

It's what I use for Christmas carp (before decapitation), and rats. If you're uncertain of your whacking tool proficiency, get a .22, a strong bb gun, rivet/dowel gun, or something similar, and take it outside.

Prepare a breadboard, plastic foil, and a tenderizer or hammer of decent mass. Check your target: enter image description here Pull the fish out of the bowl, put it between 2 layers of foil to prevent any splash, smack the brain hard. Done.

If you feel like mourning, mourn the creatures that die of natural causes without such mercy, and live through worse trauma daily.

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  • The feeling of suffocation involves light-headedness and then passing out. I would personally prefer that to a bullet to the head. And would definitely prefer it to blunt force trauma - there's always the risk that not quite enough force was used, or that aim was slightly off. That being said, with an aquarium fish, it's likely that a hammer will completely crush it.
    – DoubleDouble
    Commented Dec 3, 2015 at 17:15
  • I also do not think that suffocation is pleasant way to die. What i've been told: smash it (its head) between two rocks. It's an ultra fast death. Also size may be a factor.
    – br4nnigan
    Commented Dec 4, 2015 at 15:28
  • This is certainly not a painless way to euthanise rats, or indeed fish. As well as the issues DoubleDouble brings up, the trauma beforehand is in itself enough reason not to call this humane.
    – Piper
    Commented Dec 4, 2015 at 16:49
  • @piper I think you are forgetting "the trauma beforehand" in the other cases, as well. I once tried to "humanely" kill a rat stuck in a pipe by dropping in 2 drops of chloroform. It looked terrified in its final struggle. Chemical detection aside, many animals recognize the light-headedness as wrong.
    – kaay
    Commented Dec 6, 2015 at 11:36

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