Flatworm Exit in Canada?

[badfish!]

I've gone through 2 bottles of it treating my tank, I'd do one treatment and see the little buggers die, then a week later I'd do another treatment (even though I couldn't see any worms, I wanted to get the babies), then another treatment a week later, totalling 4 treatments. I'd figured I'd gotten them all, and for a while I didnt see any. A couple of weeks ago, I noticed they've returned though.