I use full RO water with RO right in all my tanks, but my cichlid tank. Water in my area comes out of the tap at 7.5 and runs up to 8.0 after sitting and is harder than solid rock. Also, it comes out with high phosphate levels out of the tap, but I forget what it was, since I checked it like a year ago. I have had no problems with PH bouncing around and have never had a crash due to too much C02. Of course, I use DIY C02 in all my tanks except the 75 gallon, which I use a Carbo Plus in, so you could say there isn't a problem with excessive C02. As for the GH, KH thing, RO right sets your KH at a reasonable level compared to your GH. I have my tanks at medium soft according to the RO right directions and the KH tested out at about 3.5, two days ago. If I were running my C02 at 25 PPM, that might be a problem, but since I am running it at closer to 10PPM, it works great.
Two 1 GA desktops, a 5 gallon, a 2.5 gallon, a 10 gallon, a 20 gallon, a 25 gallon, and two 75 gallon tanks, all planted.
I am an expert on algae, so ask me if you have questions. I know how to grow it, just not how to get rid of it.