Pool chemistry,
without the guesswork.

Free chlorine (FC) is the active sanitizer—it kills bacteria, algae, and oxidizes contaminants. Use liquid sodium hypochlorite, cal-hypo, or dichlor to raise FC. When shocking for algae, you typically raise FC to ~20 ppm for a short period.

High FC can bleach swimsuits and irritate skin/eyes. Lower it via sunlight exposure or partial dilution.

pH measures acidity/alkalinity of the water. Low pH increases chlorine activity but corrodes surfaces and equipment. High pH reduces chlorine effectiveness and encourages scaling.

Raise pH with soda ash (sodium carbonate). Lower with muriatic acid or sodium bisulfate. Make changes gradually and retest after full circulation.

Total alkalinity buffers pH—it resists rapid pH swings. Too low and pH bounces around unpredictably; too high and pH becomes hard to lower.

Raise with baking soda (sodium bicarbonate). To lower, add measured muriatic or dry acid, then aerate to bring pH back up.

Calcium hardness is the dissolved calcium level. Too low risks corrosion of plaster and metal parts; too high causes scale buildup and cloudy water.

Vinyl liner pools need less calcium (150–225 ppm) since the liner itself doesn't corrode—too much hardness just causes scaling. Gunite and plaster pools need more (200–400 ppm) because soft water will actually leach calcium out of the plaster, pitting and roughening the surface over time.

Raise with calcium chloride (add in stages). To reduce, partial drain & refill is the standard approach.

Cyanuric acid (stabilizer) shields chlorine from UV degradation. Without it, chlorine burns off rapidly in sunlight. Too much CYA weakens chlorine's effectiveness—sometimes called "chlorine lock."

Raise by adding granular cyanuric acid. To lower, partial drain & refill is the only reliable method.