Found this info which seems to go along with what a lot of folks have said on this thread.
Natural means of control include: Wrasse of the genus Pseudocheilinus, such as the Six Line Wrasse (P. hexataenia), Halichoeres Wrasse, such as Yellow Wrasse (H. chrysus), Macropharyngodon Wrasse such as the numerous Leopard Wrasse (Macropharyngodon sp.), and Dragonets such as the Spotted Mandarin (Synchiropus picturatus). The Blue Velvet Nudibranch (Chelidonura varians) is very effective at consuming flatworms in the aquarium, but is very sensitive to changes in water chemistry and has a short life expectancy in the home aquarium.