Why the dose is a starting point
pH is buffered by total alkalinity. At TA 120 the same acid dose moves pH less than at TA 60. The figures here assume a typical TA of 80 to 120 per 10,000 gallons. Add half the amount, run the pump for an hour, retest, and add the rest if needed. If your TA is over 120, fix that first with the alkalinity calculator.
Lowering pH
Muriatic acid is cheapest and adds nothing to the water. Pour it slowly into the deep end away from ladders and lights, with the pump running. Dry acid is easier to handle but adds sulfates, which build up over time and attack plaster and metal.
Raising pH
Soda ash raises pH with only a small bump in alkalinity. If pH keeps drifting up on its own, you can also simply aerate the water: a fountain or waterfall running for a day pushes pH up with no chemicals.