Im setting up a FO saltwater tank in august, and Im beginning my research. Correct me if im wrong, but what i have learn so far is that for saltwater tank, the water must be virtually pure so the specific gravity measured is accurate, so the amount of marine salt added is correct. And since tap water can contain impurities, a RO unit is needed to purify the water. I have also heard that if the tap water quality is good, you don't need one. Problem is, im not sure how good the tap water quality should be. Here is the annual report of the water quality in my area (I live in HK, and most of the water are supplied from Mainland China): http://www.wsd.gov.hk/filemanager/en/content_135/dongjiang_a-e.pdf the Nitrates are 2.0ppm on average, quite low.... bt im not sure about other impurities.