Domakihub How to Fix Chlorine Lock in Your Pool
Restore active sanitiser and safe water when stabiliser levels have gone too high.
Reviewed by Domakihub pool pros · 2026-06-23 · 5 min read
Chlorine lock happens when cyanuric acid (CYA) — the stabiliser that protects chlorine from the Greek sun — accumulates to a level where it binds almost all of the chlorine and prevents it from killing bacteria or algae. The video above by Swim University explains the chemistry clearly; the steps below give you a practical action plan you can follow on any residential pool, holiday rental, or hotel facility in Greece. Acting quickly matters: locked-up chlorine offers almost no sanitation protection, even though your test kit may still show a chlorine reading.
What Causes Chlorine Lock?
Chlorine lock is driven by excess cyanuric acid (CYA). Most stabilised chlorine tablets — the 200 g pucks almost universally used in Greece — contain both chlorine and CYA. Every tablet you dissolve adds a small dose of stabiliser. Because CYA does not break down on its own or get consumed by sunlight, it slowly builds up over consecutive seasons unless water is partially replaced.
The safe range for CYA is 30–50 ppm. Once levels climb above 80–100 ppm, the ratio of bound chlorine to free active chlorine tips sharply. Your test kit reads a combined figure and may still show 2–3 ppm 'chlorine', yet virtually none of it is available to sanitise. Bathers, algae, and bacteria are all paying the price.
How to Confirm You Have Chlorine Lock
The classic sign is a pool that reads adequate chlorine on a standard test strip or comparator but still clouds up, grows algae, or fails a DPD test that distinguishes free chlorine from combined chlorine. The most direct diagnostic is a CYA (stabiliser) test: use a turbidity tube or send a water sample to a pool-supply lab. A CYA reading above 80 ppm alongside persistent sanitation problems points firmly to chlorine lock.
A second tell-tale: add a large shock dose — at least 200 g of calcium hypochlorite per 10 m³ — and retest 30 minutes later. If the chlorine reading barely budges, the stabiliser is sequestering it almost instantly.
5 Steps to Fix Chlorine Lock
- 1
Test and record your CYA level
Use a cyanuric acid test kit or take a 500 ml water sample to your local pool supplier. Write down the exact CYA reading alongside your free chlorine and total chlorine figures. You need these numbers before deciding how much water to dilute or drain, because overdilution wastes money and underdilution fails to solve the problem. Target: bring CYA below 50 ppm.
- 2
Partially drain and refill the pool
Dilution is the only reliable cure. Calculate the fraction of water you need to replace: if CYA is 120 ppm and your target is 40 ppm, you need to replace at least two-thirds of the pool volume. Pump or drain from the deep end, then refill with fresh mains or borehole water. In Greece, municipal water typically contains no CYA, so every litre you add dilutes the stabiliser proportionally. For very large hotel pools, plan this as a staged process over two or three days to avoid structural stress on the shell.
- 3
Switch from stabilised to unstabilised chlorine
While the water level is low and during the refill phase, switch your chlorine source to unstabilised calcium hypochlorite (granules or liquid) or sodium hypochlorite. These add pure chlorine without adding any CYA. Do not put any more stabilised tablets (trichlor or dichlor) into the pool until your CYA reading returns to the 30–50 ppm range. This step is critical: continuing with stabilised tablets during recovery undoes the dilution work immediately.
- 4
Shock the pool with a high dose of non-stabilised chlorine
Once the pool is fully refilled, add a breakpoint shock dose of calcium hypochlorite — typically 200–300 g per 10 m³ — dissolved in a bucket of pool water and poured around the perimeter with the pump running. Aim to push free chlorine to at least 5–10 ppm. Run the filter for 24 hours continuously. Retest after 24 hours: free chlorine should now hold and the water should begin to clear.
- 5
Balance all parameters and return to a CYA-safe maintenance routine
After the shock clears, rebalance pH (7.2–7.6), total alkalinity (80–120 ppm), and calcium hardness (200–400 ppm). If CYA is now in the 30–50 ppm window, you can return to stabilised tablets — but reduce tablet use or switch to a slow-dissolve non-stabilised option in summer. Test CYA every 4–6 weeks and plan a 20–30 % partial water change at the start of each season to prevent stabiliser creep.
Rather have a pro handle it?
Get a vetted pool pro in your city — one number, booked in EN/EL/DE.
FAQ
- Can I use a chemical to lower CYA without draining the pool?
- No proven chemical product reliably removes cyanuric acid from pool water. Products sold as 'CYA reducers' have inconsistent results in independent testing. Partial draining and refilling with fresh water is the only method that pool chemistry professionals universally recommend.
- How long does it take to fix chlorine lock?
- Once you complete the dilution and refill, recovery typically takes 24–48 hours after a breakpoint shock. Larger pools (hotel or villa pools above 100 m³) may need an extra filter run cycle. You will see the water clearing visibly as active chlorine begins doing its job again.
- Will chlorine lock happen every year if I use stabilised tablets?
- It can, particularly in pools that do not lose much water to splash-out or rain dilution. The best prevention is testing CYA at the start of each swimming season, replacing 20–30 % of the pool water before summer, and keeping a log of how many tablets you add per week so you can track stabiliser input over time.