Water chemistry
Chlorine lock
Chlorine lock is a state where very high cyanuric acid makes chlorine work poorly.
Chlorine lock is a loose name for a real problem. When cyanuric acid gets too high, it ties up chlorine so it cannot kill algae fast. The test may show chlorine, yet the pool still turns cloudy or green.
The usual cause is months of trichlor or dichlor use. Adding more chlorine will not solve it. The fix is to test CYA and replace part of the water with fresh water. Then switch to liquid chlorine to stop the build-up. Ask a pro to run a full test if you are unsure.
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