I have tried FlatwormExit (before I had any fish or corals)... it worked well but didn't get them all. IME you will need to dose multiple times with larger doses each time for a decent chance of getting them all. If I had to do it over again I doubt I would mess with it.
I have a sixline... it eats a few.
Between siphoning with water changes, additional flow, lowing nutrients, a sixline, and sucking out a few I can reach with a turkey baster during the week it seems to keep the population to an unnoticeable level.