Help! New tank + newb = no clue ;)

I'm terrible at keeping things short but I'm going to try my level best. I had a small 20 or 30 gallon many years ago. I had no clue what I was doing; probably did everything wrong....but had great beginners' luck. Fast forward more years than I care to mention and my eldest daughter brought home 2 guppies from a VBS this past August. We set up a small 10 gallon for her and got her a few more fish and some live plants and it's doing fine. We had some losses but it's only after cleanings and water changes...so I've gone back to my foolish ways of old and I remove all fish during the cleaning and we're back to happy (and live) fish. Thinking the losses were due to the siphon stirring up too much waste from the gravel? In any case, I was jealous of her little tank and prayed for my own as buying something wasn't in the budget. And God heard me and within a couple weeks, my neighbor offered me his 120 gallon out of the blue. Full set-up too! I cleaned it, cured my own driftwood that I found at a reservoir, filled it with blasting grit, water and said driftwood and let it sit for about a month. I added water from our koi pond to get a bioload and when levels appeared decent, I introduced some rosies just to test the tank. They lived for weeks before I opted to move them to my daughter's tank (I wanted to add more substrate and plants to the 120 G). Months passed and I didn't touch it. This past weekend, I finally added the additional grit, buried Fluorish tabs, planted some anubias, anacharis, swords, DHG, java fern, moss, etc. I'd still like to get a lot more plants...especially some Giant hair grass and Pennywort if those are low tech friendly but baby steps.... I'm fairly new to all this considering my tank as a teen was neon gravel with plastic plants but hey...I love the look of planted tanks and everyone needs to start somewhere! I was thinking of picking up a few test fish to throw in there today if the levels were in range so my husband tested the water while I took the kids to their dentist appts. And unfortunately, it's WAY off...to the point that I'm not really sure how to correct it. Ammonia, nitrates, and nitrites are all at 0...which I expected; there aren't any fish. pH is at 8...high and that concerns me a bit. My daughter's 10 G is typically in the mid 7 range so I suspect that's partially our tap water but decent enough that as long as the other levels are fine, our fish seem happy enough. However, my husband says our kH is at 89.5 and our gH at 214.8. And that concerns me. I'm wondering if the blasting grit is responsible? SInce it's techinically coal slag and full of minerals and such, is that giving me the high #s? What can I do (as naturally as possible?) to bring those numbers down? We do have an RO filter...was thinking of emptying out 30% of the tank and replacing with RO water to see if it made a difference. Could adding wite vinegar also help to bring the pH down? And once back in range, would another 30% change and replacing with koi pond water give me enough of a bioload to introduce new fish? I've read through a bunch of stuff but since my brain is having a difficult time wrapping itself around this whole gH and kH thing and how that might affect the pH and so I'm sitting here stumped. I thought you might know best!