Hey Gord,
As you know I am by no means an expert, as I am just now researching reef aquariums for future construction. I have however done some research into flatworms and here are a couple of links reguarding them. Maybe you may find something useful to combat the little pests.
http://www.animalnetwork.com/fish2/aqfm/1998/sep/wb/
http://www.wetwebmedia.com/flatwrmfaq2.htm