Chlorine won’t rise: the 3 causes and their fixes
You add chlorine and the test stays stubbornly low? Three possible explanations: a stabiliser level too high that "locks" the chlorine, water that consumes it faster than you add it, or, on the contrary, a lack of stabiliser letting the sun destroy it.
In short
Chlorine that will not rise almost always has one of these three causes: a stabiliser level above 70 mg/L, which leaves chlorine present but inactive (only fix: renew part of the water); a high chlorine demand (algae, chloramines) consuming it instantly (fix: shock treatment); or an absence of stabiliser letting UV destroy it in full sun (fix: adjust the stabiliser).
Cause 1: the stabiliser locks the chlorine
Above about 70 mg/L of cyanuric acid, chlorine stays in the water but no longer acts: this is the most common case when stabilised chlorine (tablets) is used for a long time. No product lowers the stabiliser: you must renew part of the water, then prefer an unstabilised chlorine.
Causes 2 and 3: high demand or UV
If the water is loaded (early algae, chloramines, heavy use), it consumes chlorine as fast as it is added: a shock treatment resets things. Conversely, with no stabiliser at all, the sun destroys chlorine within hours: a stabiliser around 30 mg/L protects it.
What to do, step by step
Measure the stabiliser
This is the first reflex. Above 70 mg/L, chlorine is locked: that is your answer.
If stabiliser too high: renew the water
Partially drain to dilute the stabiliser, then top up with fresh water.
If stabiliser correct: shock the pool
A high demand (algae, chloramines) is treated with super-chlorination, with filtration running.
If stabiliser is zero: adjust it
Without stabiliser, the sun destroys chlorine. Raise it towards 30 mg/L to protect it.
Frequently asked questions
How do I know if my stabiliser is too high?
With a stabiliser (cyanuric acid) test. Above 70 mg/L, chlorine becomes ineffective even at a correct dose: it is the most common cause of chlorine that will not rise.
Is there a product to lower the stabiliser?
No, not reliably. The only truly effective method is to renew part of the water to dilute the stabiliser.
Should I stop using chlorine tablets?
If your stabiliser is already high, yes, temporarily: tablets add more. Switch to an unstabilised chlorine while you bring the level back in range.