Water Chemistry
The dissolved calcium content of pool water. Illinois IDPH accepts 150–1,000 ppm; operators target 200–400 ppm for plaster pools.
Calcium hardness is what keeps pool water from being corrosive. Too little and the water pulls calcium out of your plaster, grout, and tile — etching them over time. Too much and calcium precipitates out as scale, fouling filters, coating heater elements, and leaving white buildup at the waterline. Chicago-area municipal water is calcium-heavy, so most local pools trend upward season over season. The only way to reduce calcium hardness is partial drain and refill with fresher water.
Water softeners don't fix pool hardness — they exchange calcium for sodium, which pool water doesn't need.
`Hardness` in pool terms is calcium hardness specifically. Total hardness is a different municipal-water reading.
A scale problem often looks like a chlorine problem (cloudy water). It's not.
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Related terms · Water Chemistry
Cyanuric Acid (CYA / Stabilizer)
A pool additive that protects chlorine from UV degradation in outdoor pools. Essential in moderation, counterproductive above 70ppm.
Chlorine Lock
A state in which chlorine test readings appear normal but the chlorine is ineffective at actually sanitizing the pool. Usually caused by excessive cyanuric acid.
Combined Chlorine (Chloramines)
Chlorine that has already reacted with organic contaminants and lost most of its sanitizing value. The primary cause of the strong `chlorine smell` in pools.
Langelier Saturation Index (LSI)
A composite calculation (pH + temperature + calcium hardness + alkalinity + CYA adjustment) that measures whether pool water is corrosive, balanced, or scale-forming.
Aqua-Guard runs certified commercial pool operations for 200+ Chicagoland HOAs, condos, and clubs. We handle the credentials so your board doesn't have to.
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