Do you have visible algae that’s just not giving in to the shock you’re adding? Does your pool test show a high CYA reading of over 50ppm? Do you have cloudy water that just won’t clear up? Are you seeing purple stains or plaster degradation?

If you’ve seen some of these signs, your pool water could be experiencing something called “overstabilization.” You might also hear this pool problem referred to as “chlorine lock.”

Here, we’ll not only tell you the solution for overstabilization or “chlorine lock,” but also how to avoid it in the future.

What is overstabilization or “chlorine lock”?

Overstabilization of your pool water means that there’s a build-up of something called cyanuric acid (also called CYA) in your pool water.

How does overstabilization happen?

Usually overstabilization happens if you’ve been using stabilized sanitizers and stabilized shock products over an extended period of time, and without monitoring stabilizer levels to make sure they stay in the range of <90 ppm.

Why is overstabilization bad?

Overstabilization will significantly decrease the effectiveness of chlorine for killing germs, bacteria and algae. That means the bacteria in your pool isn’t responding to the shock and chlorine products you’re putting in it, and your pool is unhealthy for swimming. In fact, the use of stabilized sanitizer together with stabilized shock products can even cause the formation of algae when the cyanuric acid levels reach 70 ppm – this can occur within 6 to 7 weeks.

How do I know if the stabilizer (CYA) level of my pool is correct?

The recommended levels of Stabilizer (cyanuric acid) are between 40 and 60 ppm. Test your water weekly with HTH® Multi-purpose 6-Way Test Strips to find out what your CYA level is in your swimming pool.

Are there sanitizing products or shock products that do not cause overstabilization (“chlorine lock”)?

Yes. There are different formulas of shock and sanitizers. Some formulas have CYA which can build up and cause overstabilization. Calcium hypochlorite or “Cal-hypo” formulas do not contain CYA and do not cause overstabilization.

Calcium hypochlorite-based products (read the active ingredient on the label) are cyanuric acid-free and will not cause overstabilization. Several state health departments have eliminated or banned the use of cyanuric acid-based product in public swimming pools. Ensure you practice responsible pool care and have a reliable and balanced pool care program.

How do I fix my overstabilized pool water? How do I fix “chlorine lock”?

The only way to reduce CYA levels is to drain some of the water from your pool and then refill it with hose water. You do not need to empty all the water from your pool at once. Just drain 10% at a time as not to damage your pool surface or float your liner. You may need to do this in shifts and keep testing the CYA levels until they return to the proper range. Then, make sure you switch to a calcium hypochlorite (cal-hypo) shock formula like HTH® Super Shock! – calcium hypochlorite products do NOT contain CYA and will not overstabilize your pool.

Is it okay to use a sanitizer that has CYA together with a shock that does not contain CYA?

Yes. Small levels of cyanuric acid do serve a purpose in protecting chlorine from sunlight degradation. However, too much will negate any benefit and cause problems. For sparkling clear results that avoid overstabilization, use a healthy balance of HTH® Super 3” Chlorinating Tablets with a weekly dose of HTH® Super Shock!

As always, you can bring all your pool questions and concerns to our HTH® team at 1-866-HTH-POOL. They’re here to help!