Mister E if you use RO water you need your Cal Mag. I personally wouldn't be without either. I always use 50% Cal Mag and 50% nutes (ppm=300....150ppm CalMag and 150ppm nutes) and have had excellent results. I use it at 50% of total ppm for the entire grow. This may be more (or less) than others recommend but I won't change it. Ever. So there.