Skyrocketing PH

I can't sort out why my PH jumps to nearly 9 overnight! Using RO water, no algae, 5 gal. buckets in HT stones. I am new to this area, could it be hard or soft water? Would Calcium or Mag supps help? I'm at the end of my rope with this. I've nuked everything with hydrogen peroxide to kill any bacteria, I just can't sort it. My PPM meter is broken, so I'm not sure about my PPM. Any thoughts?