tinytoons right definitely use cal mag plus. But it also depends on were you live. Do you live in a industrial city or were the air quality is not always the best. You could end up with rain water thats to acidic aka acid rain. check the Ph it should be 7 and under 10 ppm. I don't know what is in the 10 ppm? so I just use a hydro-logic RO 100 then start with pure 0 ppm 7.0 Ph water and bring the cal-mag plus up to 250 ppm. No more guessing