Six Numbers That Keep Your Pool Safe — And What to Do When They Go Wrong

Your pool water looks perfectly clear. That doesn't mean the chemistry is right.

Water chemistry problems rarely announce themselves until they've been building for days. The homeowners who stay ahead of those problems don't have a chemistry background — they just know six numbers, what range each belongs in, and how to respond when something drifts.

Free chlorine is the active sanitizer in your water — the form available right now to kill bacteria, algae, and pathogens. Keep it between 1 and 3 parts per million. Below 1 ppm, your pool is essentially unprotected. Above 3 ppm, you're spending more than necessary and can irritate swimmers' eyes and skin.

Salt pools still use chlorine — the salt cell converts sodium chloride into free chlorine. The target range is identical.

pH is the most important number in your pool and the one that moves most often. It measures how acidic or alkaline your water is on a 0–14 scale, and the ideal target — 7.4–7.6 — closely matches the pH of the human eye.

Low pH (below 7.2) makes water corrosive: it etches pool surfaces, attacks equipment seals, and irritates swimmers. High pH (above 7.8) cripples chlorine. At a pH of 8.0, only about 20% of your chlorine is actually active. You can test a fine chlorine reading and still be swimming in poorly sanitized water.

Bring pH down with muriatic acid or sodium bisulfate. Raise it with soda ash. Always add chemicals with the pump running and wait an hour before retesting.

Total alkalinity (TA) is pH's stabilizer. It buffers the water so a hard rainstorm or a crowded weekend doesn't send your pH swinging overnight.

Low alkalinity makes pH erratic — you adjust it one day and it's off again 48 hours later. High alkalinity (above 150 ppm) locks pH stubbornly in place, making it hard to lower when you need to. Raise TA with sodium bicarbonate. Lower it with muriatic acid in measured increments.

Because TA and pH interact directly, adjust alkalinity first. Getting TA right makes pH far easier to hold.

Calcium hardness measures how much dissolved calcium is in your water. Too little, and soft water pulls calcium from wherever it can find it — your plaster finish, tile grout, and concrete surfaces. Over time, that causes etching and pitting that shortens the life of your pool's interior.

Too much calcium (above 400 ppm) deposits scale on tile, coping, and inside equipment.

Raise calcium hardness with calcium chloride. Lowering it requires a partial drain-and-refill — there's no chemical shortcut. In the Carolinas, municipal water runs at moderate hardness levels, so buildup over the season is the more common concern than starting too low.

Cyanuric acid (CYA) is sunscreen for your chlorine — without it, UV rays rapidly degrade free chlorine in outdoor sunlight. CYA slows that breakdown significantly.

The catch: too much CYA causes chlorine lock. Above 80–100 ppm, CYA binds chlorine so tightly it becomes largely ineffective, even when your test reads a normal level. The only way to reduce CYA is a partial drain-and-refill.

Standard trichlor tablets include CYA, so it accumulates naturally over the season. Test it monthly, not just at opening. If CYA keeps climbing, switching to liquid or granular chlorine stops the buildup.

Salt cells need a specific concentration to generate chlorine efficiently. Most systems target 2,700–3,400 ppm — far below the level you'd taste in the water. Low salt forces the cell to work harder and produce less chlorine. High salt stresses the cell over time.

Use plain non-iodized pool salt. Check your system's manual for the exact target, since different manufacturers set slightly different ranges.

Conditions shift fast. A summer rainstorm can drop your pH a full point in hours. A busy weekend party can spike your chlorine demand overnight. Two short tests per week with a liquid reagent kit — not test strips, which lose accuracy after opening — takes about five minutes and catches most problems before they're visible.

Automation systems can test and dose automatically, but even those benefit from a weekly manual check to catch what sensors can miss.

If pH and alkalinity are both off, fix alkalinity first — it's the stabilizing anchor. If your water is hazy and chlorine is depleted, shock to 10 ppm, run your pump continuously for 24 hours, and retest before letting anyone swim.

If you're chasing numbers that won't stay put, look for root causes: CYA built too high, unusual mineral content in your fill water, or a pump undersized for your pool volume. That's the point where a second set of eyes saves you hours of guesswork.

Every pool we build at Rock Water Pools includes a proper chemical startup and a hands-on walkthrough so you understand your water from day one — not guessing after the fact. If you're planning a new build or have questions about an existing pool, give us a call at 704-450-1023. The first conversation is always free.