High CYA and “Chlorine Lock”

When CYA is very high, a “normal” free chlorine reading includes more bound, slower-acting chlorine—so algae and cloudy water can persist until you dilute CYA and raise FC to an appropriate level for that stabilizer.

Why it happens

CYA protects chlorine from sunlight but ties up a fraction of FC in less active forms. Past a certain point, you’re adding large amounts of product for modest gains in real sanitizing power. Trichlor-fed pools often accumulate CYA year over year. See CYA stabilizer explained.

What to do

• Test CYA; if very high, plan partial drain/refill where allowed.
• Follow modern FC/CYA guidance for outdoor pools—don’t rely on a single ppm number without context.
• Brush, filter, and address algae protocol if present—sanitizer alone may not clear established blooms quickly.

Safe ranges / timing

Work within label limits; after major drain, rebalance alkalinity and pH before fine-tuning FC. Re-test after circulation.

Calculator

CYA Calculator · Chlorine Calculator