Pool chemistry has a reputation for being complicated, and it isn't. There are five numbers that matter, they interact in ways you can actually reason about, and once you understand what each one is doing, you stop guessing at the store and start making decisions. Everything else is noise.
What follows is the working model I give homeowners after we hand over a new pool. Learn it and you'll spend less money on chemicals, keep your plaster looking new longer, and stop chasing problems you created yourself last week.
Free chlorine vs. combined chlorine
Free chlorine is the chlorine still available to kill things. That is the number you care about, and it generally belongs in the 1 to 3 ppm range for a residential pool. Combined chlorine (chloramines) is chlorine that already reacted with sweat, sunscreen, urine, and organic debris. It is spent. It sanitizes poorly and it is what actually causes red eyes and that harsh "chlorine smell."
This is the single most useful thing to internalize: a pool that reeks of chlorine does not have too much chlorine. It has too much combined chlorine and not enough free chlorine. The fix is not to back off, it's to shock the pool — raise free chlorine high enough to break the chloramines apart. Total chlorine minus free chlorine equals combined. If combined is creeping above roughly 0.5 ppm, shock it.
pH: the number that controls everything else
pH generally belongs between 7.2 and 7.8, and the practical sweet spot is right around the middle. It matters for two reasons. First, chlorine's killing power falls off as pH climbs, so a pool sitting at 8.0 with "fine" chlorine readings is sanitizing far worse than the test strip suggests. Second, pH drives whether your water is corrosive or scale-forming.
Expect pH to drift up. Aeration does it — every waterfall, deck jet, spillover, and swimmer cannonball drives carbon dioxide out of the water and pushes pH higher. If you added water features, you signed up for more acid demand. That's not a defect, it's physics.
Total alkalinity: the buffer
Total alkalinity, generally targeted around 80 to 120 ppm, is pH's shock absorber. It's the water's capacity to resist a pH swing. Get alkalinity right and pH becomes stable and boring. Get it wrong and you'll fight pH forever.
- Alkalinity too low: pH bounces on every rainstorm and every chemical addition. This is "pH bounce," and no amount of pH adjuster fixes it. Fix alkalinity first.
- Alkalinity too high: pH climbs relentlessly and refuses to come down. You'll add acid constantly and wonder why nothing sticks.
Always set alkalinity before you chase pH. Adjusting them in the wrong order is the most common self-inflicted wound in pool care.
Calcium hardness and the Langelier balance
Calcium hardness, generally in the neighborhood of 200 to 400 ppm for a plaster pool, is where the surface of your pool is at stake. Water wants to be saturated with calcium. If it isn't, it will go get some — and on a plaster, quartz, or pebble pool, the calcium is in your finish.
The Langelier Saturation Index ties pH, temperature, calcium hardness, and alkalinity together into one number describing whether water is balanced, aggressive, or scaling.
- Aggressive (calcium-hungry) water etches plaster, roughens the surface, dulls the finish, and eventually exposes aggregate. It also eats at grout and can pit metal. This damage is permanent — you don't rebalance it away, you resurface.
- Scaling water deposits calcium as a crusty white film on the waterline tile, on the plaster, and, if you have a salt system, on the cell plates, where it kills chlorine production.
You want to be in the neutral band. Vinyl liner and fiberglass pools are far more forgiving on calcium because they have no calcium to give up, but you should still keep pH and alkalinity in range. On plaster, this is the parameter that quietly decides how many years you get out of the surface, which is often the difference between coasting and paying for a resurface earlier than you planned.
Cyanuric acid: chlorine's sunscreen
Unprotected chlorine burns off fast in North Carolina summer sun. Cyanuric acid (CYA, stabilizer, conditioner) binds chlorine loosely and shields it from UV, dramatically extending how long it survives in an outdoor pool. Generally 30 to 50 ppm is the target for a chlorine pool; salt systems typically want it a bit higher — check your manufacturer's spec.
The catch, and it's a big one: CYA does not leave on its own. It isn't consumed and it doesn't evaporate. Trichlor tabs and dichlor shock both add CYA every time you use them, so a pool run on tabs all season creeps upward, year after year. At high CYA, more of your chlorine is bound and unavailable, and the free chlorine reading on your test kit overstates how much sanitizing power you actually have. That's "CYA lock." You get algae in a pool that tests fine.
The only real fix is dilution: drain some water and replace it. Prevention is smarter — use unstabilized liquid chlorine or cal-hypo for shocking instead of dichlor, and test CYA a few times a season rather than assuming.
A realistic testing cadence
- Two or three times a week in season: free chlorine and pH. These move constantly and they're the ones that cause trouble.
- Weekly: total alkalinity, plus total chlorine so you can back out combined chlorine.
- Monthly: calcium hardness and cyanuric acid. Slow movers, but a drifting CYA is what turns July into a nightmare.
- After any big event: heavy rain, a Triad thunderstorm that dumps debris, a pool party, or a run of hundred-degree days. Test, don't assume.
Use a drop-based test kit. Strips are fine for a quick pulse check but they are not accurate enough to make dosing decisions on, particularly for CYA and calcium.
Why a new plaster pool needs a careful start-up
Fresh plaster is chemically alive. For the first several weeks it cures, sheds calcium hydroxide into the water, and drives pH up hard. If you fill and walk away, you get permanent cosmetic damage baked into a brand-new surface: mottling, streaking, scale, or a rough, etched finish. This is not a warranty issue, it's a start-up issue, and it is entirely avoidable.
A proper start-up means filling continuously without stopping (a stop leaves a ring), brushing the entire surface daily to knock down plaster dust, testing and adjusting pH frequently in those first days, holding off on the salt system until the manufacturer's window has passed, and staying off the heater until the water is balanced. Any builder worth hiring hands you an explicit start-up procedure and follows up. We do it on every custom build and on every resurface, because the first thirty days determine how the finish looks for the next fifteen years.
Building new in High Point or reviving a tired pool somewhere else in Guilford County? Request a design consultation and we'll talk through the surface, the equipment, and the chemistry it will actually demand of you. Call (336) 471-0103 and we'll get on the calendar.