all i can say is that i have a 55 and a 25 plumbed together and the 25 was loaded with flatworms..
the 55 had a scooter blenny in it for over a year and i never saw a flat worm until the blenny jumped in the overflow. now they are starting to show up in the 55...
i'm pretty sure he at least kept them in check, he was always poking around the rocks and corals...