Trust me its horrible, you watch things start to die and then you are frantically working to save what you can. After that you get the stench of your now decaying tank filling the house. Siphon, siphon, and siphon again. After you have done this for a couple of weeks its probably safe to treat. I would run carbon while you siphon as well to remove any toxins released while trapping the flatworms. Now when I see a flatworm or two (it helps knowing what to look for) I get ready to treat the tank right away after sucking the visible ones out and I have had no issues. The product really works well if the amount of flatworms is minimal.
Good Luck!