Pool Water Chemistry Guide: Testing, pH, and Shocking Explained

The five numbers that matter

• Free chlorine: 1–3 ppm (sanitizer that's still active).
• Combined chlorine: below 0.5 ppm (used-up chlorine — high numbers mean shock).
• pH: 7.4–7.6 (below 7.2 corrodes metal, above 7.8 kills chlorine effectiveness).
• Total alkalinity: 80–120 ppm (buffer that keeps pH stable).
• Cyanuric acid: 30–50 ppm (UV protection for chlorine).

Once a month also check calcium hardness (175–250 ppm for vinyl, 200–400 ppm for plaster) and total dissolved solids.

How often to test

In Westchester's swim season we recommend testing 2–3 times per week — a quick strip test on Wednesday and a full liquid reagent test on Saturday. After a heavy thunderstorm, a pool party, or a stretch of 90°F+ days, test the same evening. Chemistry can shift completely in 24 hours during a hot, humid week.

Which test kit to buy

Strip tests are fast and fine for daily checks. For accurate readings, use a liquid reagent kit with a DPD-based chlorine test — not OTO, which only reads total chlorine. The Taylor K-2006 kit is the standard we use in the field. A digital photometer is a nice upgrade but not necessary for most homeowners.

pH is the single most important number

If you only manage one thing, manage pH. Chlorine at pH 7.5 is roughly 50% "active." At pH 8.0 it drops to 20%. That's why chlorine that looks fine on paper still lets algae grow: at high pH, most of it is chemically inert. In Westchester our pools drift upward toward 8.0 all summer because our fill water is alkaline. Plan on adding dry acid (sodium bisulfate) or muriatic acid every week.

The chlorine smell myth

A strong chlorine smell around a pool does not mean too much chlorine. It means too little. What you're smelling is chloramines — chlorine that has bonded with sweat, urine, and organic matter and become inert. The fix is more chlorine, not less. Specifically, shock the pool to break the chloramines apart. This is why an over-chlorinated pool actually smells less than an under-chlorinated one.

How to shock correctly

"Shock" means raising free chlorine to roughly 10× your combined chlorine reading — enough to "break-point chlorinate" and burn off chloramines. For a 20,000-gallon pool that usually means 2–3 gallons of 12.5% liquid chlorine, poured slowly around the perimeter with the pump running.

Shock at dusk. UV destroys chlorine — a shock dose given at noon can lose half its potency before it does its job. Never mix different types of shock in a bucket; add each chemical separately and pre-dissolved if it's a powder.

Saltwater pools

A salt system doesn't change the target numbers — pH, alkalinity, and chlorine ranges are all identical. What changes is how chlorine is generated: the cell converts dissolved salt into hypochlorous acid on demand. Two salt-specific rules:

• Cyanuric acid should sit at 60–80 ppm in a salt pool (higher than a traditional pool) because the cell produces chlorine constantly and needs more UV protection.
• Inspect the salt cell every 3 months. Scale buildup on the plates kills output. A 20-minute acid rinse restores it.

Salt pools drift more strongly toward high pH because chlorine generation itself raises pH. Expect to add acid slightly more often than a traditional pool.

Cyanuric acid — the number most homeowners get wrong

Cyanuric acid (CYA, stabilizer) protects chlorine from UV. Below 30 ppm, chlorine burns off within hours. Above 50 ppm (80 for salt), chlorine binds to CYA too tightly and effectively becomes weaker — a condition called "chlorine lock."

CYA only comes out of the pool by draining water. If yours climbs above 80 ppm — common in pools that use trichlor tablets year after year — the only fix is a partial drain and refill. In Westchester, plan a 25–50% water swap every 3–4 seasons to keep CYA in range.

When to ask for help

If numbers won't stay in range for more than a few days, something is off — usually a broken sensor, a clogged filter, or a stabilizer problem. That's a good time to bring in a service that can test your fill water, inspect your equipment, and reset the chemistry from scratch.