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. Many people will tell you alcohol is a pain free way to euthanize your fish. This is incorrect. It causes burning in the gills and is very painful for the fish to endure. Using alcohol to euthanize the fish has been described as dipping a person in gasoline to euthanize them. Don’t do it! 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...

TL;DR in Indonesian:
Siapkan bak atau akuarium lain. Cobalah menggunakan MS-222. Hindari penggunaan alkohol. Gunakan minyak cengkih secara hati-hati. Pindahkan ikan. Tunggu hingga ikan tersebut mati.