WOW slow down.........First off yuy treat with the chemical, and leave it for a period of time (as per instructions) Instructions also tell you to have activated carbon on hand, and it tells you when to use that carbon, and IIRC it also tells you its a good idea to have more water made up incase a water change is necessary if fish become stressed......Please read and follow your instructions on chemical doses or such before usuing and know that you understand them or risk killing yur livestock. IIRC it also says to suck up as many flatworms beforehand so there is less toxins in the water once they are killed off. Water change was to be done after the poison not before........why stress the fish more than necessary. I have used flatworm exit before in a tank loaded with critters and corals and nothing was killed or wiped out except for the flatworms. Its safe stuff if instuctions are followed.
Natural methods may be fine for some, but perhaps some folks do not want a wrasse intheir tank afterwards........nothing wrong with Salifert Flatworm Exit if instructions are closely followed.