Chlorine in range but green water? It's the stabiliser
It's the most frustrating situation of the summer. The strip says chlorine 1.5, "in range", but the water turns green anyway. You add tablets and it gets worse. The cause, almost always, is the stabiliser (cyanuric acid).
What it is and why it builds up
Stabiliser protects chlorine from the sun: without it, the sun destroys half of it in a few hours. The problem is that it doesn't evaporate and isn't used up.
Trichlor tablets and fast granular chlorine (dichlor) contain it. Every time you use them you add a little. In an above-ground pool treated only with tablets, in 8–10 weeks the stabiliser can go from 0 to 80–100.
Why it locks the chlorine
The more stabiliser there is, the more chlorine stays "bound" and the less it disinfects. The free chlorine the strip reads is there, but it does little. That's why the right chlorine level depends on the stabiliser:
| Stabiliser | Minimum free chlorine | To keep |
|---|---|---|
| 30 | 2 | 4–6 |
| 50 | 4 | 6–8 |
| 70 | 5 | 8–10 |
| 100 | 8 | 11–13 |
At stabiliser 100, the "chlorine 1.5" on labels is a fifth of the minimum needed.
What level to aim for
- Ideal: 30–50.
- Above 90: it needs lowering.
- With salt chlorination keep it a little higher, between 60 and 80.
Measure it once a month with a strip or kit that reads it. It costs a few euros and changes everything.
How to lower it
There's only one way: replace some of the water. No product removes it. How much to drain to get down to 40:
| Stabiliser | Water to replace in total |
|---|---|
| 80 | 50% |
| 100 | 60% |
| 120 | 67% |
| 150 | 73% |
Do it in rounds, never all at once. In above-ground pools at most half the water at a time, in in-ground pools a third: a liner pool drained too far risks the liner coming away and creasing, and with a high water table an empty pool can lift. After each round refill, retest and decide on the next.
After each round, retest everything. The shock is calculated on the new water: with less stabiliser you need far less chlorine.
Example: above-ground pool of 14,600 litres with stabiliser at 100. Without replacing water the shock would need about 3.6 L of liquid chlorine. After the first round, half the water, the stabiliser drops to 50 and about 1.9 L is enough; after the second round, even less.
How to stop it happening again
- Shock with unstabilised chlorine: liquid in a drum or calcium chlorine, never tablets.
- No tablet dispenser if the stabiliser is already above 50.
- Alternate: tablets early in the season, then liquid chlorine.
- Measure the stabiliser every month.
Let Serenpool do the maths
Serenpool knows this case: if the stabiliser is high, it tells you how much water to replace and calculates the shock on the water after the replacement.
Values calculated with the chlorine/stabiliser method used by the main pool calculators. Always retest after every step.