RO water is great for hydro and if you dont mind adding back the elements that R/O water removes. You need to add CalMag (calcium and magnesium) to your R/O water or youll find deficiencies. I use just filtered water (faucet filter) and around here it gives a 150 ppm reading and 7.0 pH and keeps the calcium and magnesium. Perfect for soil. For hydro, R/O water is a neccessity to control ppm and bacteria growth.