I would advise not to go the RO, both for cost as well as the never ending pain of it, unless you really find a ned. I have really alkaline 7.8 and hard 300+ tap water. When I started CO2 I just tried working the CO2 up very slowly, watching the PH as well as my fish. My fish are the prioities in my tank, so the effect the CO2 might have on them was the major concern. I now find that much of what I had learned about the importance of PH is bogus! African cichlids have to have high PH water is fully drilled into everybody. But I find they keep on breeding and feeling fine if the CO2 is increased slowly and they can get used to the lower PH. I found when I went from 7.8 and got down to 6.8 or less they acted "off" so I now run it at near 6-9 /7.0. I have also learned that the PH varies naturally daylight to dark so a bit of swing is not a big thing. Water change is fine. Even 50% shows no stress for my fish. Watch the fish and let them tell you when it is enough?