Jonathan,
I can't recall a time I've read about this type of flatworm becoming problematic in anyone's tank. Given the size of your tank 5 is not a concern. If you start seeing hundreds then you might want to think about removing them, but any treatment you try will probably kill a lot more of the "good guys" than the "bad guys". The general concensus on these guys is that they are either neutral or good guys, so I would vote to just let them be.
Brian