Problem Summary
Chlorine test reads high or normal, but the water remains green.
A high chlorine reading with green water is a specific pattern with its own causes — it usually means the chlorine reading isn’t reflecting what’s actually available to kill algae, or the algae is a resistant strain.
Possible Causes
Very high cyanuric acid causing chlorine lock
Above a certain stabilizer level, free chlorine loses most of its killing power even though the test strip still reads 'high'.
Combined chlorine (chloramines) instead of free chlorine
A total chlorine test can read high while free (available) chlorine — the part that actually disinfects — is low.
A resistant algae strain (e.g. mustard/yellow algae)
Some algae strains tolerate normal chlorine levels and need a specific, stronger treatment approach.
Diagnostic Steps
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Test free chlorine specifically, not just total chlorine.
A large gap between total and free chlorine points to chloramines, not a true high-chlorine condition.
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Test cyanuric acid level.
A very high reading suggests chlorine lock is limiting effectiveness regardless of dose.
Solutions
Partially drain and refill to reduce cyanuric acid
Addresses cause: Very high cyanuric acid
If stabilizer is excessively high, diluting the water is often the only practical way to restore chlorine’s effectiveness.
Shock to break chloramines, then re-test free chlorine
Addresses cause: Combined chlorine (chloramines)
Use the Chlorine Calculator to calculate a proper breakpoint chlorination dose based on actual pool volume.
When to Contact a Specialist
If free chlorine is confirmed adequate and cyanuric acid is in a normal range, but the pool still won’t clear after correct shock treatment, the algae strain may need professional identification and a targeted treatment product.
FAQ
Does adding even more chlorine always help?
Not if cyanuric acid is already excessive — in that situation, added chlorine has little extra killing power until the water is diluted.