The only time I would make a point of having a lower pH is if I was breeding or had discus. But I have kept cardinal tetras at a pH of 8.8, Angels at 8.2. My tanks usually stabilize at around 7.5 after a few months.
So I can tell you how to do it but the better question is the why?
You can go ahead and use RO water. You will need to remineralize it so there is the chemicals for that, I use equilibrium for planted tanks. I have to use a double dose of it to stop the pH from crashing in the one tank I use it in. It's a 37g tank and with weekly water change of 50% and top offs and chemicals to remineralize I figured it out to like $470 a year just for the water changes. But it is the only way I can keep my levels stable and my nitrates in a safe range for my GBR. I have really high nitrates and unstable at best water. I wouldn't do it if I didn't have too.