Pool Chemistry Basics: The 5 Numbers That Matter

Free chlorine is the active sanitizer killing bacteria and algae. Anything below 1 ppm is unsafe and lets algae start; anything above 5 ppm irritates eyes and damages liners. Target 1–3 ppm consistently.

Sun, swimmer load, and water temperature all burn chlorine fast in Miami. A pool that holds 2 ppm in the morning may be at 0.5 ppm by sunset on a hot August day if cyanuric stabilizer is too low. The two numbers work together.

pH controls how aggressive or scaling the water is. Below 7.2: acidic, eats plaster, corrodes metal, irritates eyes. Above 7.8: chlorine becomes less effective, calcium scale forms on tile and salt cells.

Salt-chlorine pools naturally drift pH upward (salt cells produce sodium hydroxide). Plan on muriatic acid dosing every 1–2 weeks to keep pH at 7.4–7.6. Chlorine pools drift less but still need adjustment monthly.

Alkalinity is pH's buffer. Low alkalinity makes pH bounce around with every chemical addition; high alkalinity locks pH high and makes it stubborn to adjust. Target 80–120 ppm.

Adjust alkalinity with sodium bicarbonate (raises) or muriatic acid (lowers). Get this right before you fight pH — pH won't stabilize if alkalinity is out of range.

Calcium hardness measures dissolved calcium. Too low (<150 ppm): water leaches calcium from plaster and tile to balance itself, etching the surface over years. Too high (>500 ppm): scale forms on tile line, heater coils, and salt cell plates.

Miami-Dade's tap water runs naturally hard (250–350 ppm calcium hardness). Doral and Miami Lakes are at the upper end. Most Miami pools never need to add calcium; they need to manage scale on the high end.

Cyanuric acid (CYA) protects chlorine from sun degradation. Without stabilizer, 90% of free chlorine evaporates within 2 hours of midday sun. With 30–50 ppm CYA, chlorine half-life extends to 8+ hours.

Above 70–80 ppm, cyanuric becomes a problem: it binds chlorine and reduces its sanitizing effectiveness. "Chlorine lock." Common cause of repeat green pools. Fix: partial water replacement (drain 30%, refill).