AquariumStocking

How to Humanely Euthanize a Fish

Published October 3, 2026. Built by Jeremy Panasuk.

Quick answer: The AVMA Guidelines for the Euthanasia of Animals: 2020 Edition say personnel must be trained and that veterinarians with the relevant expertise should be consulted. Flushing a fish, or putting an unanesthetized fish in a freezer, is unacceptable. This page names the methods a private-practice veterinarian may use for a companion fish. It does not give a mixing recipe.

Check the tank before you decide the fish cannot be saved

A fish that is lethargic and off its food, in a tank set up within the last several weeks, matches new tank syndrome in the Merck Veterinary Manual: ammonia, nitrite, or both are often high enough to be toxic. That is a water problem, and it is described on how to fix cloudy water in a new tank and how to treat sick fish. Run the load before you treat the fish as beyond help.

10 gallons, normal setup, 12 small fish: 12 points, capacity round(10 × 1) = 10, percent round(12 / 10 × 100) = 120, overstocked.

Those points are this site's planning estimate for a cycled freshwater aquarium, not a medical finding. Enter them on the stocking calculator. An overstocked result means the tank itself can be what is hurting the fish. It does not forbid euthanasia when a veterinarian says the fish is suffering from something the water change will not fix. It does forbid skipping the test.

Who performs it

From the guidelines, section S6.1.2: because fish differ so much in physiology and anatomy, the best method varies, and "virtually all methods require that personnel be carefully trained and monitored." "In all cases, veterinarians and others with expertise relevant to the species of interest should be consulted." Chemicals regulated by the EPA can only legally be used according to their label directions. Some drugs require DEA registration. The guidelines also say there are currently no drugs approved by the FDA for euthanasia of fish or aquatic invertebrates.

Preparations, section S6.1.3, should be very similar to preparations for anesthesia. If possible, withhold food for 12 to 24 hours. The environment should be as quiet and nonstimulatory as possible, with light reduced but still adequate for the people doing the work. Water quality should be similar to the water the fish came from. Those are conditions for a trained person. They are not steps for a kitchen counter.

Clients may be present. The private-practice section says clients should be offered that chance when it is feasible, and should be told what method will be used and what they may see. The excitement phase of anesthesia can increase movement or look like agitation even when it is not pain. People who stay need to hear that before it happens.

Methods the guidelines name for a companion fish

Section S6.2.6.1 covers veterinary private practice for companion and ornamental (display) fish. Methods it lists as acceptable in that setting are immersion in solutions of buffered MS 222, buffered benzocaine, isoflurane and sevoflurane, quinaldine sulfate, and 2-phenoxyethanol, and injections of pentobarbital, ketamine followed by pentobarbital, a combination of ketamine and medetomidine followed by pentobarbital, and propofol followed by pentobarbital. Owners should be told that ketamine can cause muscle spasms during induction. This page stops at the names. Concentrations and milligrams per kilogram stay in the guidelines and with the veterinarian who is using them.

Acceptable with conditions, in that same private-practice list: immersion in eugenol, isoeugenol, or clove oil. "Fish should be left in the solution for a minimum of 10 minutes after cessation of opercular movement." Earlier, on eugenol, the guidelines say: "Whenever possible, products with standardized, known concentrations of essential oils should be used so that accurate dosing is possible." A spice-jar clove oil is not that product. This page does not publish a home dilution.

Immersion overdose in general: fish should be left in the anesthetic solution for a minimum of 30 minutes after opercular movement stops. A study cited in the guidelines found that buffered MS 222 as a one-step immersion was inadequate for goldfish (Carassius auratus), a hypoxia-tolerant species. The guidelines say a two-step method may be required for goldfish and some other hypoxia-tolerant species, including some cichlids: immersion to produce unconsciousness, then an adjunctive method such as decapitation, pithing, or freezing to finish. That second step is for an animal that is already unconscious, done by someone trained to do it. It is not a household instruction, and freezing is not the first step.

Not recommended in the private-practice setting: immersion in CO2-saturated water, because some fish become hyperactive, which is distressing for staff and owners; and manually applied blunt force trauma to the head, decapitation, and pithing, because applying them can be distressing for owners and staff. Rapid chilling in 2–4°C water is a conditional method only for small-bodied tropical and subtropical fish in the research discussion. The guidelines say it "is not appropriate for temperate, cool, or cold-water–tolerant fish, such as carp, koi, goldfish, or other species that can survive at 4°C and below."

What the guidelines call unacceptable

Section S6.2.4, unacceptable in any situation:

The following are unacceptable methods of euthanasia in any situation. Flushing of fish into sewer, septic, or other types of outflow systems is unacceptable for many reasons. Water chemistry and quality may delay time to death and result in exposure to noxious compounds. For systems in close proximity to and/or connected to natural waterways, pathogen release or transmission may occur from diseased or carrier animals. Slow chilling or freezing of unanesthetized animals, including placing fish into a freezer without prior anesthesia, is also an unacceptable method. Similarly death by anoxia and desiccation after removal from the water or by anoxia in water; any death due to exposure to caustic chemicals; and death including prolonged traumatic injury prior to unconsciousness are unacceptable.

Line-break hyphens in the PDF were closed so the sentences can be read. The wording is the guideline's.

How death is confirmed, and what happens to the body

Section S6.1.4: loss of movement, loss of reactivity, and initial flaccidity can indicate death. More useful for many fish are respiratory arrest — cessation of rhythmic opercular activity — for a minimum of 30 minutes, and loss of the eye roll, the movement of the eye when the fish is rocked from side to side. The heart can keep contracting after brain death, so a heartbeat is not proof of life. Sustained absence of a heartbeat is a strong indicator of death. Secondary methods are recommended, when appropriate, after the fish is anesthetized, to be sure death has occurred.

Section S6.1.5: any euthanized fish should be promptly removed from the aquarium, pond, or other vessel and disposed of according to federal, state, and local regulations, in a way that reduces disease spread and keeps other animals from reaching the remains.

A common mistake

Flushing the fish, or putting an unanesthetized fish in the freezer. Both are in the unacceptable paragraph above. So is taking the fish out of the water and leaving it to dry. The other mistake is skipping the 120% stocking check and the ammonia test, and deciding the fish needs euthanasia when the tank is the emergency.

Limits

This page is not a protocol you can carry out from the screen. The guidelines require trained personnel and a consult with a veterinarian. If you are unsure whether the fish is suffering from water quality, infection, or an injury that will not recover, that question is also the veterinarian's, after the tests on how to treat sick fish. The Merck management chapter says veterinarians should follow these AVMA guidelines. There is no product link on this page.

Compiled with AI assistance from the sources below. Spot an error? Email jpanasuk@gmail.com.

Sources