The issue with either RO water or RO/DI water is that the filtering process stripes everything from the water, leaving you with (essentially) H2O. Living animals (especially aquatic ones) relie heavily on water to provide them with essential vitamins and minerals that they need. In a saltwater tank, these stripped necessities are replaced via the sea salt mix we add back to the RO/DI water. In the case of a freshwater system just using RO or RO/DI water means that the organisms under our care are not going to receive the vitamins and minerals that they need. So long as you are will to buy the replacement minerals using RO water or even RO/DI should be fine, and in your case so long as you are dosing you should be fine. In my case, my tap water falls well within the acceptable range, so rather than spending the money to dose a whole regiment of replacement supplements, I simply dechlorinate and use tap. While I've seen that it seems to be becoming more and more of a popular choice, I'm not sure that many are really doing the research they should. Many seem to simply be adding the RO water without dosing. Additionally, I've done a little research (granted its not been a lot) and to be honest, I haven't found anything that seems to replace all the required elements in one shot like the salt mix does for salt tanks. Out of curiousity, what are you dosing to replace the required elements?
In this particular case, I'll be using RO water in the misting system, but for the PWCs, I'll be using dechlorinated tap. That way I'm not losing elements the fauna requires.
Yep, the ion brick absorbs ions so until the substrate reaches capacity, it changes the pH.
Thanks, definitely going to get a few more orchids in there as well as some others.