Water Parameters That Actually Matter

Clear, stable pool water is not a mystery; it is chemistry applied with purpose. We walk through the seven parameters that truly control balance: pH, total alkalinity, calcium hardness, temperature, total dissolved solids, cyanuric acid, and the chlorine you measure to protect swimmers. Industry “ideal ranges” help, but they often leave a wide gap that causes confusion in day-to-day care. That is why we focus on target values inspired by Bob Lowry’s approach, then validate the interplay with the Langelier Saturation Index using tools like the Arenda app. When you aim at specific targets instead of broad ranges, you correct faster, stabilize sooner, and stop chasing your tail after every heatwave or rainstorm.

Start with pH because it drives comfort, sanitization efficacy, and LSI. A steady 7.5 is practical and lands right where chlorine is effective without pushing the water toward corrosion or scale. Pair that with total alkalinity around 90 ppm to buffer pH without forcing it upward all week. If your pH drifts high, drop TA to 80 ppm to slow the rise. Calcium hardness must reflect your surface: around 350 ppm for plaster to protect finish integrity and 250 ppm for vinyl or fiberglass to reduce scale risk. Temperature is not optional data; warm water boosts scale formation potential and speeds chemistry, so plug it into the LSI with pH, TA, and calcium before deciding what to dose next. TDS matters primarily as a ceiling for aging water, but remember salt systems raise TDS by design, so treat TDS as context, not a red flag.

Cyanuric acid (CYA) is where most care plans fail. CYA shields chlorine from sunlight, but it also binds chlorine and slows its kill rate. That is why a static “3 ppm chlorine” rule breaks down at high CYA. Lowry’s simplified guidance is to keep free chlorine at 7.5 percent of CYA for reliable sanitation. At 100 ppm CYA, you need about 7.5–8 ppm chlorine; at 50 ppm CYA, 4 ppm chlorine does the job. Keep CYA at 50 ppm when possible to maintain effective free chlorine without extreme dosing and to make midseason control easier. If your route or household depends on stabilized tabs, watch the creeping CYA and plan partial drains before water quality slips and algae takes hold despite “high” chlorine.

Borates change the equation in your favor. At about 50 ppm, borates act as an algistat, disrupting algae development so chlorine is not fighting a constant bloom. They buffer pH, slowing the weekly climb that eats acid and time. Because borates improve chlorine’s staying power, you can use a 5 percent FC-to-CYA ratio and maintain strong protection with less swing. The visual bonus—crisper sparkle—is real because borates improve water clarity and feel. Practical sourcing like boric acid powder keeps application simple, and levels around 30–40 ppm still help, though 50 ppm is the sweet spot for performance. Always confirm local guidelines and handle chemicals safely, but do not overlook how much labor and product borates can save over a season.

Tie it all together with the LSI so you avoid scale on tile lines and corrosion in heaters. Input temperature, pH, TA, calcium, CYA, and TDS into a reliable calculator before making large corrections. Nudge pH toward 7.5, keep TA near 90, and set calcium to match your surface and climate. Hold CYA around 50 for residential control, then align free chlorine to 7.5 percent of that number—or 5 percent when borates are present. These targets reduce guesswork, prevent surprise algae at “high” chlorine, and protect surfaces and equipment. When your parameters support each other, you stop reacting and start maintaining, week after week, with clear, resilient water.