Bob Lowry on Dialing in Free Chlorine the Right Way

Pools that “won’t hold chlorine” are one of the most common and expensive headaches in pool maintenance, especially for pool service pros trying to stabilize water fast. When someone says they’re adding two or three gallons of liquid chlorine a day to an 8,000 to 10,000 gallon pool and still reading near zero free chlorine, the first move is not panic dosing. It’s investigation. Chlorine demand usually has a cause: organics, contaminants, or active growth. Start by looking beyond the water test and into the yard, the deck, and the daily habits around the pool. Grass clippings, fertilizer drift, landscaping debris, and rain runoff dripping from dense vegetation all add organic load that chlorine must oxidize. Even well-meaning actions like washing the deck so dirty rinse water flows into the pool can wipe out chlorine overnight.  

Once you suspect contamination, think like a detective and list every source of oxidizer demand. Heavy bather load can create big swings, especially when a backyard pool becomes the neighborhood hangout. Sweat, cosmetics, and sunblock enter the water and immediately compete for free chlorine. There’s also the “unknown unknown” factor: someone rinsing tools in the pool, solvents and cleaners, or accidental chemical mixing. Many of these compounds consume chlorine aggressively and can keep the pool stuck at zero despite constant dosing. On the biological side, remember algae does not appear overnight at full strength. By the time you see green dust on steps or haze in the deep end, the growth is already established. Biofilm in plumbing or on surfaces can also create persistent chlorine demand that never seems to end until it’s fully oxidized.  

That’s where the idea of negative chlorine demand matters. Sometimes you add a large dose, test shortly after, and only see 1 to 2 ppm free chlorine when you expected 10 to 20. The missing chlorine didn’t “vanish” into thin air; it got consumed bringing the pool back up from a deficit caused by organics or growth. A practical remedy is to raise chlorine to a high target and hold it there long enough to finish the job. The approach discussed is to bring free chlorine up to about 25 ppm or 40% of cyanuric acid (CYA), whichever is higher, then keep testing and re-dosing so the level does not crash. Holding that elevated free chlorine for 24 to 48 hours, with circulation and brushing, helps oxidize contaminants and kill algae instead of letting chlorine get partially consumed and never complete the cleanup.  

If your tests show “okay” chlorine but algae still shows up, the missing piece is cyanuric acid and active hypochlorous acid (HOCl). Only a small fraction of free chlorine is in the most effective killing form, and that fraction drops further as CYA rises. That’s why a fixed industry target like 2 to 4 ppm can fail in real pools. A more reliable rule of thumb for algae prevention is keeping free chlorine at roughly 7.5% of your CYA level. With 50 ppm CYA, that means closer to 4 ppm free chlorine, not 2. Consistency matters too: if free chlorine dips below the needed level for even a day or two, algae spores can take advantage of sunlight and available nutrients and bloom quickly. For better efficiency, reduce the bather contamination load with a simple rinse before swimming, which can dramatically cut chlorine consumption and help keep free chlorine steady between service visits.