Is this a whole-house system, or are there bathtub taps, etc. that aren't filtered? There are various mixes to re-constitute pure RO water (RO Right, Equilibrium), but I have tried them and wound up with some funky algaes. RO water may need additional buffering to prevent pH swings. If you have very hard tap water, you can experiment with mixing RO and straight tap water until you get the conditions you want.