I use RO/DI only, but then again my water is kinda scary. I looked up the water content online and came up with a list of nice little additives already in my local water like aluminum, fluoride, uranium, copper, lead, chloride, maganese, sulfate, boron, calcium, chlorine, magnesium, phosphate & radon. The way I look at it is I wouldnt want to drink it so I dont want my fish and corals to have to breath it. Plus when I think about the kind of investment I have in my reef tank the last thing I want to do is loose anything due to poor water quality.