Unless all the fish in a tank need to be euthanized, you'll need a second container. Transfer some of the water from the current tank to a new, clean container, preferably with a similar aeration and temperature control setup.Using water from a different source or at a different temperature can stress the fish, or cause an inhumane death.


Some drugs, including MS-222, may not be effective in temperatures below 10ºC (50ºF).
Euthanizing a large tank of fish is difficult, and requires measurements of oxygen and chemical concentration for best dosage calculations.Ask a veterinarian to perform this if at all possible.
 Also sold as tricaine methanesulfonate, "Finquel," or "Tricaine-S" at pet stores, this is the most reliable option of the widely available drugs. You can find this at pet supply stores.This is the only widely available drug approved by the FDA for this purpose.Use 5–10 times the amount the label recommends for anaesthesia, usually 250 to 500 mg/L.Do not eat fish killed this way.
 Another option is 95+% ethanol, available at laboratory supply stores and some hardware stores. Use 10–30 mL ethanol for each liter of water (2–6 tbsp per quart), at the same temperature as the tank water.Vodka will work as well, if you increase the amount based on the strength. For example, 63% vodka requires 1.5 times as much by volume to reach the same alcohol concentration.
 It's not easy to tell how concentrated a given bottle of clove oil is, or even what exactly it contains. It may put your fish to sleep but fail to kill it. The fish will not be safe for eating, and the water may cause harm if dumped in the water supply.If you decide to use it, try this method:Shake one drop in water until milky, then add to the tank.
When the fish is asleep at the bottom, shake and add another 13 drops per liter of water (50 drops per gallon).
 Once you've added the chosen drug, net the fish and quickly move it to the euthanasia bath. Handle as little as possible to avoid causing stress.
 Any euthanizing drug can send the fish to sleep if too little is used. Death often takes 30 minutes to arrive, and you may need to wait 2 hours to be sure.Watch carefully for the following signs of death:No movement of gill flap for 10 minutes. (Usually following spasms 1 minute apart.)
No movement of eye when fish is rocked from side to side.
Very slow heart rate. The heart may continue after death, but a strong, persistent beat means the fish is alive.
If you do not see these signs within an hour or two, or if the fish wakes up again, add more of the drug.
If you want to be certain the fish is dead, kill it with the physical methods below, or freeze it in ice water. These should not cause pain if the fish is deeply anaesthetized.
Prepare a separate tank.
Consider using MS-222.
Try alcohol instead.
Use clove oil cautiously.
Transfer the fish.
Wait until the fish is dead.
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