CliniPool / Understand your water
Stabiliser (cyanuric acid): the friend that can sabotage your pool
- Stabiliser (cyanuric acid) shields chlorine from UV — without it, chlorine vanishes within a few summer hours.
- BUT it accumulates: every stabilised chlorine tablet adds some, and it leaves almost only through a water change.
- Too much stabiliser = "locked" chlorine: the test shows a good level, but the water is no longer protected.
- It's the number one hidden cause of "inexplicable" green water in late summer.
Stabiliser — cyanuric acid, “cya” to its friends — is chlorine’s sunscreen: without it, UV destroys an outdoor pool’s chlorine within a few hours of high summer. Essential, then. But it has a flaw few people know about: it accumulates. Every stabilised chlorine tablet adds a little more, and it leaves almost only through a water change. Too concentrated, it “locks up” the chlorine: your test shows a superb level while the actual sanitisation collapses. It’s the hidden cause behind countless “inexplicable” green pools at the end of the season. The CliniPool app measures it and factors it into every dose calculation.
Chlorine’s sunscreen
Unprotected free chlorine is extremely vulnerable to UV. Stabiliser forms a protected reserve with it, released gradually. An outdoor pool without stabiliser burns through an absurd amount of chlorine; with a sensible level, consumption becomes normal. So far, so good.
The accumulation trap
The problem comes from how it arrives: most chlorine tablets and powders (dichlor, trichlor) are stabilised chlorine — so every treatment adds stabiliser. And stabiliser doesn’t wear out, doesn’t evaporate, can’t be filtered: it stays. Season after season, tablet after tablet, the level climbs. And the higher it climbs, the smaller the genuinely active share of chlorine at any given reading: at high levels, your “2 ppm of chlorine” on the test amounts to almost nothing in real sanitising power.
The classic signature: late summer, water turning green, chlorine tests “fine”, shocks with stabilised chlorine achieving nothing (and adding yet more stabiliser!). Plenty of pool owners live through this for years without anyone explaining it.
Getting out of it: the only real method
Bad news, owned: no product reliably lowers stabiliser. The only method is dilution: drain part of the water and top up with fresh. The proportion is simple to calculate — it follows the gap between your level and your target — and the app gives it to you with your rescue plan. Then, to avoid ending up back here: switch part of your chlorination to unstabilised chlorine (hypochlorite), or dose the stabilised kind with your eyes open. Shocks, in particular, are best done unstabilised.
The lesson to remember
Stabiliser is the most underestimated parameter in pool care: invisible, with no direct symptom, it sabotages in silence. Measure it a few times a season (it moves slowly), especially if you chlorinate with tablets. The app tracks it over time, warns you when the build-up approaches the critical threshold, and picks, for every treatment, the type of chlorine that won’t make things worse.
Frequently asked questions
How do I lower a stabiliser that's too high?
There's only one reliable method: replace part of the water (partial drain, then fresh top-up). Stabiliser doesn't evaporate and can't be filtered out.
Why does my water turn even though the chlorine is fine?
With a very high stabiliser, the measured chlorine is largely inactive: the test counts "locked" chlorine. The water can therefore turn green with a perfect-looking reading. That's the signature of over-stabilisation.
Which products contain stabiliser?
"Stabilised" chlorine: tablets and powders based on dichlor or trichlor (check the label). Hypochlorite (bleach, unstabilised chlorine) and salt electrolysis add none.
What's the right stabiliser level?
The usual ballpark sits around 30 to 50 ppm for an outdoor chlorinated pool; beyond 70-100 ppm, chlorine effectiveness degrades markedly. The app interprets your level in the context of YOUR pool.