easies way i found is to isolate the fish and fill the container with hydrogen peroxide the fish will slowly start swimming less n less. its best compairable to the "shot" givin to dogs and cats
Thank you for your comments, everyone - unfortunately, by the time I was able to get back to my parents' place, they'd relocated the fish to their pond in hopes of giving it more room to thrash safely and it jumped out, too far away to flop back in. It died the next morning. -.- Well, at least the poor thing is finally at peace...
I just had to put a four inch goldfish down. put him in a gallon or water and add 3 drops of clove oil to a cup of wather and shake it up really good then add itt o the fish. aftr about 20 mins you can add three more drops the same way and after 15 mins or so 3 more oil drops the same way. then after about an hour if he is still breathing add 3 more drops of oil and after about a half an hour he shouold no longer be breathing. then you can bury or flush. but the bigger the fish the longer it takes so you might have to add three more drops at a time slowely over time.