pH
Also called: pool pH.
pH is the measure of how acidic pool water is. Aim for pH 7.6–7.8 and treat 7.2–8.0 as acceptable, which is Trouble Free Pool's recommended level and is the same for vinyl, plaster and fiberglass, and for liquid-chlorine and salt pools.
Why it matters
Pool pH almost always drifts up, and that is normal: pool water holds more dissolved carbon dioxide than the air above it, and as CO₂ escapes, pH rises toward a natural resting point around 8.2. Anything that stirs the surface speeds it up. The 7.4–7.6 on test-kit bottles is a public-pool figure, written for water with no stabilizer. Below about 7.2 the water turns aggressive toward plaster and metal; above about 8.0 scale forms more easily. Acid lowers pH and alkalinity together. Aeration raises pH and nothing else.
In numbers
Don't correct a 7.7 that's on its way up. A salt pool with alkalinity at 60 can be left to drift to 7.8–8.0 before you add acid.