I was worried about flatworms a couple of years ago when they seemed to be exploding in the tank. But after reading wetwebmedia, I just left them alone and now they are gone. (They said the population cycles though booms and busts and they never recoved from the last bust) Of course I do have a tank full of damsels, a 6-line wrasse and all manner of wild things in there.
I have since seen the same thing happened to my encrusting sponges, tiny red feather duster worms, micro brittle stars, beach hopper amphipods, 2 different species of snails and lastly the mighty bristle worms (which my fish are now trained to eat)