Your pool has chlorine but the water is still green? Stabiliser may be the problem

Think of cyanuric acid (or stabiliser) as sunscreen for your pool's chlorine.

• Without it: the sun "burns off" the chlorine within hours.
• With the right amount: it protects the chlorine so it keeps disinfecting for longer.
• With too much: the shield gets so thick it locks the chlorine up completely. The chlorine is there, but it can't attack algae or bacteria. It's "asleep".

This is called chlorine lock, and it's the main reason many pools go cloudy even when the test says "you have chlorine".

🚧 The trouble with "all-in-one" tablets

Stabiliser doesn't appear by magic. It gets into your pool mainly through chlorine tablets (trichlor) and fast-dissolving granular chlorine (dichlor).

The golden rule: stabiliser goes in, but it NEVER evaporates.

If you use only tablets for years without replacing any water, stabiliser keeps building up until the chlorine stops working.

🚦 Traffic-light check: how is your water?

To find out, measure cyanuric acid (CYA) with test strips or a dedicated test kit.

• 🟢 30–50 ppm (ideal): your chlorine is protected and active. All good.
• 🟡 50–90 ppm (warning): chlorine starts losing strength. You need much more of it to do the same job.
• 🔴 Over 100 ppm (danger): chlorine is locked. Algae are very likely and the water may not be safe, however many chemicals you add.

🛠️ The fix: how do I bring it down?

The bad news: there's no magic chemical that removes it. The only real solution is dilution:

1. Measure the exact level.
2. Drain part of the pool (usually between a third and a half, depending on how high it is).
3. Refill with fresh, clean water.

💡 3 tips so it doesn't happen again

1. Don't rely on tablets alone: alternate with liquid chlorine (hypochlorite) or unstabilised shock, which disinfect without leaving a residue that builds up.
2. Replace some water: backwash the filter regularly and, at least once a year, replace part of the pool volume.
3. Test before you treat: don't add chemicals blindly. Check stabiliser at least once a month during the swimming season.