Need advice to lower PH

My shrimp tank's PH shot up to just below 8.0 yesterday. Had not added any tap water since 2 weeks ago. No dead schrimps (headcount taken and correct) Tap water's PH is at 7.4.

3ft Tank about 2 months already and has only black quartz sand as substrate and 1 DW with moss.

1. What could have cause the PH to goes up when tap water PH is at 7.4 ?
2. Added K-leave water (at PH 6.8) but no effect on tank's PH.
3. Wanted to add a layer of ADA Soil to sustrate to bring down PH. how do I treat the ADA to minimise PH swings when introduced into tank with cherries and CRS.

Thanks for advice.