pH, Alkalinity & Hardness

Balancing pool water is three adjustments in a fixed order, and the order matters more than the doses.

Alkalinity first. Then pH. Then calcium hardness.

Total alkalinity is the buffer that holds pH steady. Adjust pH while alkalinity is out of range and the alkalinity simply pulls pH back where it was — and you will spend a fortnight chasing your own tail with a test kit.

Target ranges

Total alkalinity 80–120 ppm. pH 7.2–7.8. Calcium hardness 200–400 ppm.

Everything drags something else with it

Sodium bicarbonate raises alkalinity and nudges pH up. Soda ash raises pH and takes alkalinity with it. Muriatic acid lowers both — it neutralizes bicarbonate, and bicarbonate is most of what total alkalinity measures.

Which is why "high pH with low alkalinity" cannot be fixed with a single chemical. The fix is aeration, which drives pH up without touching alkalinity, then bicarbonate, repeated.

Never mix, always wait

One chemical at a time, hours apart, pump running. Muriatic acid and chlorine together produce chlorine gas, and people are hospitalised by that every summer.

Retest after a full turnover — six to eight hours on most residential pumps. A reading taken ten minutes after dosing tells you about the water beside the return jet.

Move in halves

Add half the calculated dose, wait a turnover, test, add the rest. Overshooting alkalinity or hardness is far harder to undo than undershooting, because both come down only by draining and refilling.

Hardness is a one-way street

It never leaves the water except by dilution, and too much of it scales the heater, the tile line and the filter. If you are at the top of the range already, do not add more "for safety".