How to lower chlorine in a pool
Chlorine that is too high usually comes down on its own within a day or two once you stop adding it. Before doing anything, check whether it really is too high: the right level depends on your stabilizer, and a reading that looks high for one pool is normal for another.
Is it actually too high?
Stabilizer (cyanuric acid, CYA) holds most chlorine in reserve, so pools with more CYA need more free chlorine to do the same job. A widely used guideline is free chlorine of at least 7.5% of CYA, and never below the CDC minimum of 2 ppm for stabilized pools.
| Stabilizer (CYA) | Minimum free chlorine | Shock level (40% of CYA) |
|---|---|---|
| 30 ppm | 2.3 ppm | 12 ppm |
| 50 ppm | 3.8 ppm | 20 ppm |
| 80 ppm | 6 ppm | 32 ppm |
So 6 ppm is high for a pool with 30 ppm CYA but only the minimum for a pool with 80 ppm CYA. The chlorine and stabilizer guide explains why. Even so, follow the label of your chlorine product: it may give a maximum level for swimming.
Check the test too. The CDC notes that DPD tests (the pink-color kind) can bleach out above about 10 ppm and read falsely low or zero. A strip that reads off the chart, or a DPD sample that flashes pink and then clears, means the level is high and you need a test that reads higher, such as an FAS-DPD drop kit.
Ways to lower chlorine
1. Stop adding chlorine
Take tablets out of the floater or feeder, turn down or pause a salt chlorine generator, and don't add any more liquid chlorine. In most outdoor pools, chlorine falls by a few ppm a day on its own. How fast depends on sunlight, temperature, swimmers and stabilizer level.
2. Use sunlight
Remove the cover. Ultraviolet light breaks down chlorine, especially in pools with little stabilizer. Pools with high CYA lose chlorine more slowly, because stabilizer protects it from the sun.
3. Run the pump and aerate
Circulation and splashing spread the chlorine evenly and help the water lose it to sun and air. This also stops a "hot spot" near where a tablet or dose was added from giving a misleading reading.
4. Dilute with fresh water
Draining part of the pool and refilling lowers chlorine in proportion. It's slow and uses a lot of water for chlorine, which drops on its own anyway, so it makes most sense when you also need to lower stabilizer, calcium or salt. The drain and refill calculator works it out.
Example: to lower free chlorine from 12 ppm to 5 ppm with fill water that has no chlorine, you'd replace (12 − 5) ÷ (12 − 0) = 58% of the water. Waiting a few days is usually the better choice. Municipal water often contains some chlorine of its own, so test your fill water and enter that reading.
5. Use a chlorine neutralizer
Products based on sodium thiosulfate react with chlorine and remove it quickly. They're useful when you need the pool back fast, but they're easy to overdo: too much leaves the water unable to hold chlorine until the leftover neutralizer is used up. Follow the product label for the dose, add a fraction of it, circulate, and retest.
What not to do
- Don't add other chemicals to "cancel" chlorine. Never mix chlorine with acid or any other pool chemical. See pool chemical safety.
- Don't drain more than you need. Some pools can be damaged by lowering the water far. Check your builder's or liner maker's advice.
- Don't neutralize a shock in progress. If you raised chlorine to clear algae, removing it early lets the algae come back. Finish the shock first: see how to shock a pool.
Why chlorine ends up too high
- A salt chlorine generator set higher than the pool needs, especially in cooler weather or with the cover on.
- Too many tablets in the floater, or a feeder set too high.
- A dose worked out for the wrong volume. The pool volume calculator checks it.
- Chlorine added on a schedule rather than in response to a test.
After a high reading, check pH once chlorine is back in range. High chlorine can make some pH tests read falsely high.
Frequently asked questions
Is it safe to swim with high chlorine?
Follow the label of the chlorine product you used: it sets the level for swimming. Very high chlorine can irritate eyes and skin, and it can fade swimsuits. If you just shocked, see when you can swim after shocking.
How long does it take for chlorine to go down?
There's no single answer. A sunny, uncovered pool with low stabilizer can lose several ppm in a day. A covered pool or one with high stabilizer can hold its level for days. Test daily and watch the trend.
Does a strong chlorine smell mean too much chlorine?
Usually not. The sharp "chlorine" smell comes mostly from chloramines, which form when there isn't enough free chlorine. See strong chlorine smell.