Total alkalinity

Raising it

Baking soda. 1.5 lb raises TA 10 ppm in 15,000 gallons. Spread it over the surface and brush it in. It nudges pH up a little, which is usually fine, since low TA and low pH tend to show up together. What to buy and what to carry: starter chemicals.

Lowering it

Nothing lowers TA without also lowering pH. Acid drops both. So you use that on purpose:

1. Add acid and bring pH down to 7.0–7.2, lower than you'd normally take it
2. Aerate: run the features, jets, a spillway, anything that stirs the surface
3. Aeration drives off CO₂, which brings pH back up and leaves TA down
4. Repeat over a few visits until TA lands in range

This takes several visits. Trying to do it in one dose overshoots pH badly, and on plaster you risk etching. On vinyl, remember the 7.2 floor and take smaller steps.

Total alkalinity is what keeps pH from moving. Get it right and pH stays where you left it. Get it wrong and you're either chasing pH every week or watching it swing.

Two ways it goes wrong

That last row gets misread a lot. If you're adding acid every week and pH is high again seven days later, the problem is TA. You're treating the symptom. Fix the TA and the pH sorts itself out.

TA and LSI

LSI uses carbonate alkalinity, not the total. The math subtracts about a third of your CYA reading, because stabilizer shows up in the TA number without doing the same buffering job. A pool reading TA 100 with CYA 90 is really running closer to 70. On high-CYA salt pools that gap is big enough to move the LSI into a different band. See reading the LSI.