Pool chemistry can feel like it requires a lab technician. It doesn't. There are six numbers that matter, they interact in predictable ways, and understanding the logic behind them makes your pool easier to manage and your service reports more meaningful.
The Sanitizer: Chlorine
Chlorine kills bacteria, viruses, and algae. It exists in two forms in your water: free chlorine (FC) and combined chlorine (CC). Free chlorine is the active sanitizer. Combined chlorine is chlorine that has already done its job and bonded with contaminants — it's technically still chlorine but does almost nothing useful. Total chlorine is simply FC + CC. Your service report shows all three.
The Foundation: pH
pH controls how effective chlorine is. At pH 7.4–7.6, chlorine operates at roughly 55–60% efficiency. At pH 8.0, it drops to around 20%. This is why a pool can test fine for chlorine but still struggle with algae — the pH is too high to let the chlorine actually work. Maintaining pH in the correct range is the single biggest lever pool owners have over water quality.
The Stabilizer: Cyanuric Acid
Outdoor chlorine is attacked by UV radiation from sunlight. Without CYA, an unshaded pool in Oklahoma July can lose its entire chlorine residual in a few hours. CYA acts as a sunscreen for chlorine. But there's a catch: very high CYA levels (above 80–100 ppm) reduce chlorine effectiveness so much that you need to significantly raise chlorine levels to maintain sanitation — a phenomenon called "chlorine lock." This is why partial drain-and-refills are occasionally necessary.
The Balancers: Alkalinity and Calcium
Total alkalinity acts as a buffer that prevents pH from swinging wildly. Without enough alkalinity, pH becomes erratic — bouncing up and down with every rain, splash, or chemical addition. Calcium hardness determines how aggressive your water is toward surfaces and equipment. Soft water (low calcium) is hungry — it will dissolve calcium out of your plaster. Hard water deposits calcium onto surfaces as scale. The right balance keeps water comfortable and equipment protected.