Pool Chemistry Truths: Chlorine, pH & CYA with Eric Knight

Pool pros often inherit rules that made sense before stabilizer became standard. The idea that a low pH always creates stronger chlorine is one of those rules. In non-stabilized water, pH does control the split between fast-killing hypochlorous acid and slower hypochlorite ion. But once cyanuric acid enters the picture, that lever loses most of its power. The majority of chlorine binds with CYA to form isocyanurates, and the effective speed of free chlorine remains nearly the same from pH 7.0 to 8.0 at typical outdoor CYA levels. That means chasing a low pH for kill power in a stabilized pool wastes time, acid, and attention without the payoff many expect.

This shift forces us to separate two different goals: sanitization and balance. Sanitization is about clear, safe water with predictable chlorine performance and reliable oxidation; balance is about protecting surfaces and equipment from corrosion and scale, guided by the Langelier Saturation Index (LSI). pH matters a lot for balance because it has the largest swing on the LSI. It matters far less for sanitization when CYA is present because the FC-to-CYA ratio governs chlorine’s usable strength. If we keep conflating these goals, we end up overcorrecting pH for the wrong reason, then battling rebound as pH naturally rises toward its equilibrium near 8.2.

A smarter approach is to contain pH rather than control it. Use the LSI to set practical floors and ceilings. Raise calcium hardness or adjust alkalinity to create a buffer that slows pH drift and keeps the LSI within a safe band. Instead of forcing pH to 7.2, consider 7.6 to 7.7 as a starting point that reduces the daily climb and keeps balance steady. Meanwhile, for sanitization, target an appropriate FC-to-CYA ratio. Many experts find 15 to 30 ppm CYA optimal outdoors because it shields chlorine from sunlight while keeping the required free chlorine manageable. Above 50 ppm, maintaining the necessary FC gets much harder, and at 100+ ppm, contact times and chlorine demand can become impractical.

High CYA doesn’t just dull chlorine; it stretches contact time for tough pathogens. Public health guidance treats this seriously. When contamination events happen, pools with elevated CYA face longer closures or mandatory dilution because the chlorine present simply cannot work fast enough. Practically, that means overstabilized residential pools often need partial drains to restore a manageable ratio. There is no chemical shortcut for extreme CYA levels. Once you bring CYA into range, enhance your program with enzymes to reduce oxidant demand, and consider phosphate control to limit growth pressure, which helps your free chlorine focus on sanitation rather than burning through non-living load.

Chlorine enhancers are not magic; they are strategy multipliers. Enzymes target oils and bather waste, freeing chlorine to handle disinfection. Phosphate removers limit the nutrients algae need, lowering the baseline chlorine requirement. Secondary oxidizers like ozone can further reduce combined chlorine and improve clarity. But none of these tools replace a sound FC-to-CYA target and balanced water on the LSI. Think of the program as a chain: CYA within range, FC matched to CYA, pH contained for LSI, and supplements used to trim demand. When each link is set correctly, kill speed, clarity, surface protection, and user comfort fall into place with less chasing and fewer surprises.