that's one way of doing it i guess.
with me small fish become feeders and i treat the large ones till the last minute which sometimes pays off.
here is an small chapter out of a book i got called "starting right with goldfish" by Robert Gannon published in 1973 [DO NOT DO THE FOLLOWING!!]
Chapter 9
How To Kill Your Fish Quick
- Feed them so much u can see food lying on the bottom of the tank, moldering. In the same way, allow dead fish to stay in the tank until they disintegrate. And never syphon off excrement, of course. You can tell when things are going well if the sand becomes gray of black, if the water is cloudy and if the aquarium smells.
-if the above fails, put a few fresh-water mussels into the tank bottom. They'll die shortly and quickly pollute the aquarium
-Allow guest to flick cigarette ashes into the tank for for quick poisoning. Also, use metallic piping which will rust and corrode. Another good way to poison fish is to let a "marble" type aquarium rust around the top so brownish streaks run down to the water edges. If you have a rust -proof stainless steel tank, adjust your air releaser to splash up against the rustable light reflector.
-During the heat of summer, chill the fish by pouring ice water into the tank.
the above i thought is funny since its out of a real book but obviously all are WRONG DO NOT DO THE ABOVE what i also found funny is "releaser" and "rustable" aren't even real words