Algae keeps coming back even when your test reads “plenty of chlorine”? Here’s the missing piece: cyanuric acid quietly ties up most of your free chlorine, leaving only a small slice as active HOCl. We dig into the real chemistry behind clear water, then translate the math into one simple rule you can use on every stop.

We walk through the practical percentage target that actually prevents algae: set free chlorine to 7.5% of your CYA, or drop that target to 5% when you run borates at 50 ppm. You’ll hear why pools with 150–200 ppm CYA can still turn green at 6 ppm FC, how pH shifts HOCl to weaker OCl−, and why weekly shock isn’t a strategy. If you’ve ever fought a trichlor-heavy pool through summer and wondered why “within range” fails, this conversation gives you the framework to fix it.

We also unpack LSI fundamentals to predict scale and corrosion, then explain why balance on paper isn’t enough if the pool won’t stay put between visits. Targets beat broad ranges: tune alkalinity to control pH rise, maintain appropriate calcium hardness, and let borates smooth pH swings so sanitizer stays effective. By focusing on stable set points instead of chasing numbers, you protect plaster and equipment, reduce callbacks, and keep water reliably clear.

• CYA binding lowers active chlorine and HOCl
• The 7.5% of CYA free chlorine rule
• Borates at 50 ppm reduce the FC target to 5%
• High CYA pools need higher FC despite “good” tests
• LSI as a prediction tool, not a cure-all
• Targets for stability versus chasing ranges
• Balance that holds week to week, not just on paper

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