Hi old fisher
Flat worms are a pain in the back side as they mutiply over time the best way to get rid of them are blue velvet nudibrances as they eat them do not use anytreatment to kill them off as the will toxic your tank so nudibrances are the bestway and also a 6 line wrsasse eats them but best bet on the nudibrances once there gone take the nudibrance back to the lfs as they can wipelout your tank if they die
regards scott