Pool chemistry sounds complicated. It isn’t. There are five numbers that matter, five ranges to keep them in, and a handful of decisions to make when a number drifts. Once you understand what each one does, the rest is just habit.
1. pH — The Master Number
Target: 7.4–7.6
pH controls how effective everything else is. Too low (below 7.2) and your water gets acidic — it starts eating plaster, metal, and skin. Too high (above 7.8) and chlorine becomes almost useless, plus you get cloudy water and scale.
To raise: soda ash (sodium carbonate). To lower: muriatic acid or dry acid.
2. Free Chlorine — The Sanitizer
Target: 1–3 ppm
This is what actually kills bacteria and algae. “Free” chlorine is chlorine still available to work. “Combined” chlorine is chlorine that has already reacted and is now inactive but still stinky (that classic “pool smell” is combined chlorine, not the disinfectant — it means your pool needs more, not less, chlorine).
If free chlorine drops below 1 ppm, algae start winning. Above 5 ppm and you’re wasting chemicals and irritating eyes.
3. Total Alkalinity — pH’s Bodyguard
Target: 80–120 ppm
Alkalinity buffers pH. High alkalinity means pH is hard to change (good — it stays stable). Low alkalinity means pH bounces around with every rainstorm or chemical addition.
If pH keeps drifting no matter what you do, check alkalinity first. Usually the fix is here, not with pH directly.
4. Calcium Hardness — Protects Your Surface
Target: 200–400 ppm
Water that’s too soft (low calcium) will pull calcium out of your plaster to satisfy itself — pitting and etching the surface. Water that’s too hard (high calcium) deposits scale on tile and inside the heater.
Calcium is slow to change and hard to lower. If yours is high, partial water replacement is often the only real fix.
5. Cyanuric Acid (CYA) — Sunscreen for Chlorine
Target: 30–50 ppm
The most misunderstood number. CYA (also called “stabilizer” or “conditioner”) protects chlorine from being destroyed by UV light. Without it, direct sunlight destroys 90% of your chlorine within a few hours.
But too much CYA is worse than none. Above 80 ppm, chlorine becomes so bound up that it can’t sanitize effectively — you need much higher chlorine levels to compensate. Only way to lower CYA is water replacement.
How Often to Test
- pH and chlorine: at least twice a week. More in hot weather or heavy use.
- Alkalinity: weekly.
- Calcium and CYA: monthly is usually plenty.
Use a Real Test Kit
Test strips are convenient but not accurate — pH and chlorine readings drift within seconds and the colors are hard to read. Get a Taylor K-2005 or K-2006 drop test kit ($60ish, lasts years). Or hire someone who tests properly every week (that’s us).
Free water analysis if you’d rather have someone else keep the numbers right.