In my experience with flatworms, the fish tend to eat the white ones, the nasty, toxic brown flatworms are left alone, since they are toxic. If you want to go natural, go with velvet nudi branch, if you could find one, and idiot proof every powerhead, overflow box, and whatever else can suck them in. They are all suicidal and just wait for a perfect moment to die. I promise you no one fish could put a hurt on them, they are hiding in low flow areas in your tank. Unless you just got few of them a week ago or so, you could get rid of them, past that, they will hide in every cravice of your lf. You could control them through detailed vacuuming every week or so, but it will only control them. There are chemicals, but the toxins from the fws could wipe your tank out.