Raising alkalinity
Plain baking soda is the same chemical as any "alkalinity increaser" sold for pools, at a fraction of the price. About 1.4 lb per 10,000 gallons raises TA by 10 ppm. Spread it over the surface with the pump running and retest after six hours.
Lowering alkalinity
Acid lowers TA and pH together. Add the acid in two or three stages, letting pH fall to around 7.0 each time, then aerate to bring pH back up. Aeration raises pH without touching alkalinity, so repeating the cycle walks TA down while pH ends where you want it. Use the pH calculator for the final pH nudge.
Why it drifts
Trichlor tablets and acid both consume alkalinity over a season, so tablet pools drift low. Fill water high in bicarbonate does the opposite. Testing TA once a month is enough for most pools.
Dose tables by pool size
Written-out amounts for every product: 5,000 gallons, 10,000 gallons, 12,000 gallons, 15,000 gallons, 18,000 gallons, 20,000 gallons, 25,000 gallons, 30,000 gallons.