Free chlorine (FC) is the active sanitizer, it kills bacteria and algae and oxidizes contaminants. It’s measured in ppm. Use chlorine products (liquid sodium hypochlorite, cal-hypo, dichlor) to raise FC. When shocking to handle algae you typically raise FC to much higher levels (e.g. 20 ppm) for a short period; always follow product safety and label guidance.

Ideal range: 2.5 – 5.0 ppm

Common caution: high FC can bleach swimsuits and irritate skin/eyes; lower by sunlight exposure or partial dilution if too high.

pH measures how acidic or basic the water is. pH affects sanitizer effectiveness (low pH increases chlorine activity but can corrode surfaces; high pH reduces chlorine effectiveness and encourages scaling).

Ideal range: 7.2 – 7.6

How to adjust: raise pH with soda ash (sodium carbonate); lower pH with muriatic acid or sodium bisulfate. Make changes gradually and retest after circulation.

Total alkalinity acts as a buffer for pH, it helps resist rapid pH swings. If alkalinity is too low, pH will bounce around; too high and pH can be hard to lower.

Ideal range: 80 – 130 ppm

How to adjust: raise alkalinity with baking soda (sodium bicarbonate). To lower alkalinity, add measured acid (muriatic or dry acid) and aerate to raise pH back after.

Calcium hardness indicates dissolved calcium. Low hardness risks corrosion of plaster and metal parts; high hardness risks scale formation and cloudy water.

Ideal ranges: Vinyl liner: 150 – 225 ppm; Gunite/plaster: 200 to 400 ppm

How to adjust: raise hardness with calcium chloride (add in stages). To reduce hardness, partial drain & refill is the common approach.

Cyanuric acid protects chlorine from UV degradation (sunlight). Without enough CYA, chlorine will burn off quickly; too much CYA reduces chlorine’s effectiveness.

Ideal range: 30 – 50 ppm

How to adjust: add cyanuric acid (stabilizer) in small amounts; to lower, partially drain & refill.