I get u i agree but Nature is not based on perfection, quality rather on quantity and balance ,its not made to save all fishes but the hight growth rate maintains balance as most of the fishes either die or get eaten before maturity but still enuf survive ,on theother hand we try to save all fishes in controled environment thts why we do un natural things going against natural survival of the fittest , i did experienced some situations where pond can have dissease or over population of parasites where only treating a fish wont solve problem and u cant take out all fishes in qt for a day or 2