Switching to RO water at pH 6.2 is a good move if your soil's getting too alkaline from tap water (7.9 is pretty high). In living soil, the microbial life usually buffers pH pretty well, but consistently high-pH watering can still push things out of range over time. Just using RO water alone can gradually bring things back down, but it’s slow — adding a bit of humic/fulvic acid or even a compost tea can help speed things up and support microbial activity. I’d avoid anything too aggressive unless you’ve tested the soil pH and know it’s way off. If you see signs of N excess (dark, clawing leaves), it could be tied to high pH, boosting nitrate availability. Just keep the RO water consistent, add a light organic acid, and let the soil biology do its thing.