Had a recent bout with a sudden pH increase from my water company rerouting our supply and it was a pain.
I would start by asking your water company for their annual report (good base line but it doesn't reflect seasonal and occasional changes). Test for just what it is that makes your water hard or pH high. I found my carbonate had quadrupled, so I needed an acid buffer to break up the carbonate so I wouldn't get into a pH bounce mode, so say'eth the good folks at Seachem.
What they didn't mention, and I didn't know enough to ask, was that their buffers and Neutral Regulator all have phosphate in them, so then all of my planted tanks started growing algae. Then I had to remove the phosphates, which are buffers to help stabilize the pH. Kind'a maddening. Added some more driftwood and increased water changes so things are back to normal.
If you have an extension service, university or water filtration company nearby you might try getting an analysis from them before you go sockin' the money into an R/O system. If your fish are already pretty well acclimated to mineral rich water you'll end up going through all kinds of fun to put back essential parts of whatever the R/O has removed.