Why Your Pool Chlorine Keeps Disappearing in Summer
As the days heat up, a lot of pool owners notice the same thing: the chlorinator is running, the salt is fine, but every test shows low or no chlorine. The water starts to lose its sparkle and, if it goes on long enough, algae moves in. More often than not the problem isn't the chlorinator. It's the sun.
Sunlight breaks chlorine down
Chlorine in pool water is broken down by UV light. On a clear Brisbane summer day, an unprotected pool can lose most of its free chlorine within a few hours of sun. Your chlorinator or liquid chlorine keeps adding more, but it's being burnt off almost as fast as it's made. That's why a pool can test fine first thing in the morning and close to zero by mid-afternoon.
What stabiliser does
Stabiliser (cyanuric acid) acts like sunscreen for chlorine. It bonds loosely with the chlorine and shields it from UV, so the chlorine lasts long enough to keep the water sanitised. Stabiliser doesn't get used up the way chlorine does. It stays in the water and only drops when water leaves the pool through backwashing, splash-out, leaks or heavy rain overflowing the pool.
As a general guide, most outdoor chlorinated and salt pools sit somewhere around 30–50 ppm. Some chlorinator manufacturers recommend a different range, so check your chlorinator's manual or ask us when we test your water.
Signs your stabiliser is low
- Chlorine tests low in the afternoon even though the chlorinator is set high.
- You're running the chlorinator for longer and longer to keep up.
- Algae keeps coming back after it's been treated.
- There's been a lot of backwashing, a refill or a big storm since the last test.
The only way to know for sure is a water test. Home test strips don't always include stabiliser, and it's easy to misread, which is one reason regular water testing matters.
Too much is a problem too
More isn't better. When stabiliser climbs too high, it holds on to the chlorine so tightly that the chlorine becomes slow to work, even though the test says there's plenty there. Pools in that state can go cloudy or green with a healthy-looking chlorine reading. Because stabiliser doesn't break down, the only way to bring a high level down is to partly drain and refill the pool. That's why it should be added in measured amounts based on a test, not topped up by guesswork.
Other things that use up chlorine in summer
Sun isn't the only drain on chlorine when it's hot. Warm water, more swimmers, sunscreen, leaves and storm debris all use chlorine up faster. Make sure the pump and chlorinator run long enough each day for summer, keep the skimmer and pump baskets empty, and give the pool a brush and vacuum after a busy weekend or a storm.
Adding stabiliser
Stabiliser dissolves slowly. Follow the label on the product, and pre-dissolve it or add it through the skimmer as the label directs, with the pump running so it circulates. Don't backwash for a few days afterwards or you'll send some of it down the drain. Retest after about a week before adding any more.
If the stabiliser is right and the chlorine is still low, the chlorinator itself may need attention. A scaled-up cell makes much less chlorine. See how to check and clean your chlorinator cell. If the pool has already turned, our guide to cleaning up a green pool will help.
Get it tested before summer hits
Bring a water sample into our shop at 304-306 Junction Rd, Clayfield and we'll test it for you, or let our technicians look after it as part of a regular service — see our services. Book online or call (07) 3857 8530.