When you use R/O (Reverse Osmosis) water you strip the water of everything including buffers. Which are the chemicals that resist PH swings. When you just add cal mag it adds no buffer capacity. In hydroponics there are few nutrients that will buffer your PH. Only ones being phosphates, citrates, and carbonates. Carbonates being the best choice for a buffer. Calcium carbonate and magnesium carbonate is what buffers your ground water. So given the opportunity to use straight tap water is always the best choice compared to R/O. Only use R/O if you absolutely have to.