Rookie Pool Mistakes Pt.1 – Bob Lowry

Pool pros and owners often trust the two-to-four ppm chlorine guideline, then wonder why algae returns week after week. The hard truth is that cyanuric acid (CYA) drastically changes how much active sanitizer you actually have in the water. Most of the free chlorine binds to CYA, leaving only a small fraction available as hypochlorous acid (HOCl), the form that kills algae and pathogens. At typical pH values, and even at modest CYA like 30 ppm, roughly 97 percent of chlorine is tied up. That doesn’t mean it’s useless, but it does mean only about three percent is ready to work at any moment, and pH further shifts the HOCl to less potent OCl−. This is why “I have six ppm free chlorine” can still coexist with green walls.

The fix is a practical rule that reverses the math: set free chlorine as a percentage of CYA. Rather than chasing a static two-to-four ppm, target seven and a half percent of CYA as your free chlorine level to reliably prevent algae. If CYA is 100 ppm, aim for 7.5 ppm FC. This aligns your sanitizer with the binding dynamics in stabilized water and removes the guesswork. Better yet, you can add borates at 50 ppm to reduce that target to five percent of CYA, making real-world maintenance more achievable. Borates help buffer pH movement and limit algae’s foothold, so your active HOCl can stay effective without weekly shock “rescues” that mask an underlying imbalance.

Many pros learn this lesson the hard way on high-CYA, trichlor-heavy pools where algae returns despite “high” chlorine. At 200 ppm CYA, six ppm FC is not enough; with borates, you still want about ten ppm FC to keep algae at bay. Seeing this in the field changes habits fast: measure CYA every visit, set FC targets by percentage, and stop relying on vague “okay ranges.” Your clients care about outcomes, not whether the strip says the color looks fine. That’s especially true in regions where CYA creeps up over a season and sunlight is intense, like California and Arizona. Stabilized pools demand a stabilized approach.

Beyond sanitizer control, water balance matters, but balance alone doesn’t guarantee stability. The Langelier Saturation Index (LSI) predicts whether water tends to scale, corrode, or sit near equilibrium by combining pH, alkalinity, calcium hardness, temperature, and now CYA and borate factors. It’s useful, but it can be gamed into a “perfect” zero while the pool remains unstable. For example, high alkalinity with a low pH might balance on paper, yet the pH will drift up fast, forcing constant chasing. Stability means the water holds steady between visits, with targets that keep pH creep manageable, protect surfaces, and maintain sanitizer performance. Targets, not ranges, keep you from banking on luck.

Think of it like conversation between parameters: alkalinity nudges pH upward, CYA tames but binds chlorine, borates smooth out pH swings, calcium interacts with pH and carbonate to form scale. When each piece hits its target, the whole system behaves. That’s why pros who work from well-researched targets often don’t need to compute indices on every stop. They know the set points that prevent scale, avoid corrosion, and safeguard plaster and equipment worth six figures. When a pool is stable, you stop firefighting and start preventing, saving chemicals, callbacks, and client trust. Start with a fresh CYA test, set FC to 7.5 percent of CYA or five percent with borates, tune alkalinity to temper pH rise, and verify calcium is adequate for your surface and temperature. Then watch the route get calmer and the water stay clear.