Pool stabiliser is the most misunderstood chemical in South African pool care — most owners have never tested for it, many have never heard of it, and yet it quietly decides whether the chlorine you pour every week actually works. If your pool eats chlorine and still goes green, this article is probably about your pool.
What pool stabiliser does
Stabiliser — cyanuric acid, or CYA on a test kit — is chlorine’s sunscreen. Unprotected chlorine in a Cape Town summer pool is destroyed by UV within hours; with adequate CYA in the water it survives the day and does its job. That is the whole magic: the right amount keeps your chlorine alive.
CYA numbers that matter:
- 30–50 ppm: the healthy zone for outdoor SA pools
- Below 20: chlorine burns off by lunchtime — add stabiliser
- Above ~80: chlorine increasingly locked up — efficiency falls
- 150+: the pool can be green WITH high chlorine readings — dilution time
The trap: stabiliser creep
Here is what nobody tells you at the till. The most common chlorine products in SA — stabilised granules and floater pucks (dichlor/trichlor) — carry CYA inside them. Every dose adds a little more, and CYA does not leave the water: it does not evaporate, it does not get filtered out; it only dilutes when water is physically removed. Dose a pool with stabilised chlorine for three years and the CYA quietly climbs past 100, 150, 200… and each ppm makes your chlorine a little lazier. The result is the signature call-out: “I shock it every week and it is STILL green.” The chlorine is present — the test strip says so — but it is handcuffed.
Diagnosing and fixing lock-up
- Test CYA — most home kits skip it; any decent pool shop or our assessment includes it;
- Under 80: you are fine — your green-pool problem lives elsewhere (usually filtration — see our sand filter guide when it publishes, or the repair checklist);
- Over ~100: the only real fix is partial drain-and-refill — typically a third to half the pool, sometimes staged. Yes, it feels wasteful; it is also the only thing that works;
- Then change the habit: switch routine dosing to unstabilised chlorine (liquid or calcium hypochlorite) and keep CYA where you want it deliberately instead of accidentally.
The Cape Town angle
Two local wrinkles. First, winter rain dilutes CYA along with everything else — a partial natural reset every year, and one reason spring testing matters. Second, water-conscious Capetonians hate draining, understandably — which makes preventing creep (unstabilised routine dosing) the far better strategy than curing it. And a caution before any big drain on an older pool: empty shells need respect, and marginal shells need assessment first — if a major dilution is on your cards, talk to us before pulling the plug.
Frequently asked questions
Is stabiliser the same as the “conditioner” on the label?
Yes — stabiliser, conditioner and cyanuric acid (CYA) are the same thing wearing different marketing.
Do indoor or fully covered pools need CYA?
Barely — no UV, no burn-off. Low CYA is correct there; it is outdoor SA sun that makes 30–50 ppm essential.
Can high CYA harm swimmers?
It is not meaningfully toxic at pool levels — the harm is indirect: disabled chlorine and the sanitation risk that follows.
Why does my salt-chlorinated pool also need stabiliser?
Salt cells make unstabilised chlorine — in summer sun it still needs its CYA sunscreen, or the cell runs flat out and ages early.
