Soda ash (sodium carbonate)
Also called: sodium carbonate, pH Up, pH Increaser.
Soda ash is sodium carbonate, sold at the pool store as "pH Up" or "pH Increaser". It raises pH strongly and raises total alkalinity along with it, so it is the right tool only when both numbers are low.
Why it matters
Pick the pH raiser by what you want to happen to both numbers. If alkalinity is already fine or high, aerate instead, because aeration raises pH and nothing else. If pH is low but alkalinity is where you want it, borax lifts pH with far less effect on alkalinity. Baking soda is the wrong tool for a low pH; it mainly raises alkalinity. Most pools drift up, so a low pH usually traces to heavy trichlor or dichlor tablet use, or acidic fill water or rain. Trouble Free Pool's recommended level is pH 7.6–7.8, with 7.2–8.0 acceptable.
In numbers
Soda ash is strongly alkaline, so poured in one spot it spikes the pH in a plume and calcium carbonate drops out as a fine white haze. Broadcast it slowly over the deep end with the pump running.