Heres the rig Zum14 mentioned, Looks pretty friggen sweet:
http://www.airwaterice.com/c=HoupxHM...TDS-Meter.html
Ahh here is the unit I was looking for... (though there is another one of a different brand with similar purposes):
http://www.aquariumspecialty.com/cat...oducts_id=3497
Just some sweet options to choose from but again theres tons of cheap systems. It really doesn't matter, they all do the same thing in the end. Just make sure you set it up in a way that will be as convenient as possible but also functional.
Cheers,
Chris