I use reconstituted RO water for my tanks because my tap KH is anywhere from 8 to 10 dKH (about 115 to 150 ppm). I use Seachem Equilibrium for my GH (I shoot for a GH of 5 dGH or 89 ppm) and sodium bicarbonate to set my KH (I shoot for 1.5 dKH or 22 ppm). Your guppies might like a KH more like 6 (about 90 ppm). GH in typically doesn't affect fish, only plants. I would not get hung up on actual pH, worry more about getting the proper KH. If the only thing buffering your water is carbonate, your pH will be around 7.8.
Edit: I looked back at your post and saw you measured your KH/GH in ppm, I used dGH which is about 18 ppm and dKH which is about 15 ppm.