I've never used RO. I can almost guarantee half the hydro growers here use tap water, but it all depends on your tap water. My tap is 116 ppm 6.8 ph consistently so I am successful using it. However, if you're on well water with 450ppm and ph of 9, you'll have a hell of a time keeping your plants happy. So to some degree, I do agree but only if your tap water or well water is bad