I've got a refugium full of algae and zero nitrates. My phosphates were high until I started running ROWAphos.
Then I started reading about the Redfield ratio and learned that both carbon and Nitrogen are limiting my phosphate removal. That's why a refugium will never lower your phosphates to zero unless you are willing to add carbon and nitrates to your tank. But, chances are they will use up all the available nitrates.
Here's some reading:
http://www.reefcentral.com/forums/sh....php?t=2109768