Sodium bicarbonate

Sodium bicarbonate is the gentle way to raise total alkalinity. It moves pH only a little, which is why pros use it when alkalinity is low but pH is fine.

The pool grade is the same chemical as the baking soda in your kitchen, but pool packs are cheaper in bulk. Add it in small doses with the pump on, broadcast over the deep end, then wait several hours and retest. Too much makes water cloudy and raises pH over time. Follow the product label.