Update:
I tested my tap water and it reads PH 7.6
KH 125ppm GH 143ppm.
I've also tested my tap water after leaving out for 48 hours with and without the gravel. - PH 7.6, KH 125ppm. GH 161ppm. So a slight rise in GH compared to straight out of the tap, but no further rise with gravel.
After...