How to Lower Chlorine in a Pool

Here is how to reduce pool chlorine without creating a new problem: stop adding chlorine, keep the pump running while following your pump manual’s electrical warnings, let sunlight and time bring the level down, and measure again. If you need it lower sooner, you can replace part of the water with fresh water (check local regulations before you drain) or use a chlorine neutralizer at the dose printed on its label. Before you do any of that, confirm the reading, because a very high chlorine level can make a reagent kit read low instead of high.

Chemical safety first: Pool chemicals, including chlorine neutralizers, can irritate skin and eyes and give off harmful dust or fumes. Read the label on every product and use only the amount it directs for your pool’s volume. Never mix pool chemicals with each other, never put two products in the same bucket, and never try to cancel out chlorine with an acid or a household product. Wear gloves and eye protection, and keep chemicals secured away from children and pets. This guide is general information, not medical advice. If someone is exposed to a chemical or feels ill, follow the first aid directions on the label and call a poison center at 1-800-222-1222; call 911 for emergencies.

First, Confirm the Chlorine Is Actually High

Start with a reading you can trust. The CDC’s guidance for home pool and hot tub owners says a DPD kit gives the most accurate results, and that strips are less accurate. Follow the kit maker’s directions either way.

One detail on that same CDC page matters a lot here: if the chlorine concentration of the water sample is more than 10 ppm, the reagent can partially or totally bleach out, which gives a false low or 0 ppm result. So a reading of zero the morning after you shocked the pool does not prove the pool needs more chlorine. It may mean the chlorine level is high enough (above 10 ppm, per the CDC) to bleach the reagent and give a false low result. Do not add more chlorine on the strength of that reading. Keep the pump running, following your pump manual’s electrical warnings, and measure again later. If your kit’s directions describe a dilution step for high readings, that step is the one to use.

Next, check which number is high. Free chlorine is the part still available to disinfect, combined chlorine has already reacted with contaminants, and total chlorine is the two together. The CDC’s chloramines and pool operation page, written for pool operators, gives the math as combined chlorine equals total chlorine minus free chlorine. A high total reading with ordinary free chlorine is a different problem from high free chlorine, and the steps in this guide are for the second case.

Smell is a poor guide. That same CDC chloramines page describes chloramines as a type of combined chlorine that forms in water and then off-gasses into the air above it, and it lists the distinctive chloramine odor as a warning sign. A sharp “chlorine” smell over the water is a reason to measure, not proof that chlorine is too high.

What Counts as Too Much Chlorine?

The CDC publishes minimums for disinfection rather than one target number. Its home pool guidance recommends a chlorine concentration of at least 1 ppm in pools, or at least 2 ppm if you use cyanuric acid or stabilized products such as dichlor or trichlor, with pH between 7.0 and 7.8. Its guidelines for staying healthy in pools say to follow the manufacturer’s instructions for your product and describe the typical range for chlorine as 1 to 4 ppm.

For an upper limit, the label on your chlorine product is the document that counts, and it is more than a suggestion. As one example, a chlorinating tablet label on file with the EPA states that it is a violation of federal law to use the product in a manner inconsistent with its labeling, and it says not to re-enter treated pools if the chlorine level is above 4 ppm. The same label’s directions say chlorine must be between 1 and 2 ppm. Other products set their own levels, so read the label on the product you actually used. Your pool maker’s manual can list a different range (Bestway’s frame pool manual, for example, lists 2 to 4 ppm free chlorine), so follow the label of the product you used and check your pool’s manual.

How to Reduce Pool Chlorine: Four Options

Work from the option that adds nothing to the water toward the ones that add a new chemical or new handling steps. Measure between each step so you know whether the next one is even needed.

1. Stop Adding Chlorine and Keep the Water Moving

Pull the tablet floater out of the pool, close or turn down an inline feeder as its manual describes, and take any tablets out of the skimmer basket with gloved hands. Do not put removed tablets back into their original container unless the label allows it, and do not add them to a dispenser or container that holds other product. Keep them dry and away from other chemicals, and follow the label’s directions for leftovers. The tablet label on file with the EPA calls for goggles or a face shield, protective clothing, and rubber gloves when handling the product. It warns that contamination with moisture, organic matter, or other chemicals may start a chemical reaction, with heat and hazardous gases, and says not to reseal the container in case of contamination. It also says not to add the product to a dispensing device that holds remnants of any other product, and never to place unused product down any indoor or outdoor drain. If you have a salt system, see the salt water section below. Keep the pump on its normal schedule so the water mixes and your readings reflect the whole pool rather than the spot near the return.

If you run a pump on an above ground pool, follow your pump manual’s electrical warnings. Bestway’s Flowclear pump manual, for example, warns of a risk of electric shock and says the pump cannot be used while people are inside the pool, says to make sure your hands are completely dry when plugging in the filter pump, says not to use an extension cord to connect the unit, and says to keep the ground-fault circuit-interrupter (GFCI) dry and off the ground. For an inground pool, follow the electrical warnings in the manual for your own pool’s pump.

Then keep measuring. The CDC recommends checking chlorine concentration and pH at least twice per day, and more often when the pool is being used by a number of people. That routine also shows you how fast the level is falling. This is the slowest method, and it is also the one with the least that can go wrong.

2. Let Sunlight Do Some of the Work

The CDC’s home pool guidance lists sunlight among the things that reduce chlorine levels, along with pee, poop, sweat, and dirt from swimmers’ bodies. If a solar cover is on, taking it off during sunny hours exposes more of the water to the sun. With the cover off, it is sensible to keep the gate closed and an adult watching whenever children are near the pool.

Results vary from pool to pool. Stabilizer (cyanuric acid) is generally used to slow chlorine loss in sunlight, so a pool with a lot of it may come down more slowly than one with little. A 15 ft above ground pool in full afternoon sun and an inground pool shaded by oaks can behave quite differently, which is why your own morning and afternoon readings tell you more than any rule of thumb.

3. Replace Part of the Water

Draining some water and refilling with fresh water dilutes everything in the pool, chlorine included. It works faster than waiting, but it comes with its own checks:

• Read your pool maker’s manual before draining. Bestway’s frame pool manual, for example, warns in its storage and winterization steps that improper draining of the pool might cause serious personal injury or damage to personal property. Your manual may not cover partial draining for chemistry; if it does not, ask the maker’s support line before lowering the water. For an inground pool, ask the pool’s builder or a professional pool service before you lower the water.
• Watch children while the water level changes. The safety section of that Bestway manual warns that a pool presents a drowning hazard even during filling and draining, and says to maintain constant supervision of children.
• Check local rules before you drain. The draining steps in that same Bestway manual, written for taking the pool down, say to check local regulations for drainage by-laws. Ask your local public works or stormwater office what applies where you live.
• Think about where the water goes. An EPA stormwater fact sheet, written for municipalities, says that water treated with chemicals such as chlorine harms local water quality and is toxic to wildlife and fish when it enters surface waters, and that many communities have laws or ordinances that prohibit certain discharges, including chlorinated pool water. The fact sheet lists options that municipalities can teach residents in place of sending that water to the storm sewer system or directly into local waterways, such as draining to a sanitary sewer, or dechlorinating the water before it is discharged. It also notes that local codes and ordinances vary by location; one city it cites prohibits discharging private swimming pool water into the public sewer system. Ask your local public works or stormwater office which of these options, if any, applies where you live before you drain.
• Check the inlet and outlet before the pump runs again. If you run a pump on an above ground pool, follow your pump manual’s directions on water level as well as its electrical warnings. Bestway’s Flowclear pump manual, for example, says not to dry run the filter pump and to make sure both the pool’s inlet and outlet valves are fully covered by water before operating it. For an inground pool, follow the manual for your own pool’s pump.
• Measure after refilling. Run the pump long enough to mix the new water, then check free chlorine and pH again before adding anything.

4. Use a Chlorine Neutralizer, Exactly as Labeled

Chlorine neutralizers are sold for this job. HASA’s safety data sheet for its neutralizer lists the ingredient as sodium thiosulfate pentahydrate and gives its recommended use as lowering chlorine levels in swimming pools and spas following super chlorination. The same sheet says the product causes eye and skin irritation and is harmful if inhaled, tells users to wear protective gloves, avoid breathing the dust, and use the product only outdoors or in a well-ventilated area, and lists acids and oxidizing agents as incompatible materials. Other brands may use a different formula, so check the active ingredient on the label you buy.

If you decide to use one, these steps keep the risk down:

1. Read the entire label before opening the container. Use the dose it gives for your pool’s volume and your current reading. Do not estimate or scale it yourself.
2. Add it on its own. The CDC’s recommendations to prevent pool chemical injuries say never to mix different pool chemicals with each other or with any other substance, and to open one product container at a time and close it before opening another.
3. Follow the label on how to add it. That same CDC report says to predissolve pool chemicals only when the product label directs it, and then to add the pool chemical to water, never water to the pool chemical.
4. Run the pump, wait the time the label gives, and measure again before you consider adding more. Follow your pump manual’s electrical warnings whenever the pump runs. Too much neutralizer could leave the pool with too little chlorine to disinfect, which puts you on the other side of the problem.
5. Check pH in the same reading. The CDC’s recommended pH range of 7.0 to 7.8 still applies after any treatment.

A neutralizer can bring the level down faster than waiting, and it is also the only one of the four options that brings a new chemical into the yard. If the reading is only somewhat above your label’s range and nobody needs to swim today, waiting is the simpler choice.

Lowering Chlorine in a Salt Water Pool

A salt chlorine generator keeps making chlorine for as long as the cell runs. To bring the level down, lower the output setting or the cell’s run time using the controls described in the manual, and make sure a boost or “super chlorinate” mode is not still running from an earlier shock. Some controllers may also have a reduced output mode for when the pool is covered; the manual will say if yours has one.

After one change, give the pool time to respond and measure before changing the setting again. Small changes followed by readings are easier to judge than big swings. If chlorine keeps climbing at the lowest setting, contact the equipment maker’s support line. Do not open the control box or work on cell wiring yourself; electrical work belongs with the maker’s service network or a licensed electrician.

When Can You Swim Again?

Two things decide it: the product label and a fresh reading. Wait the re-entry time printed on any product you added, including a neutralizer, then measure free chlorine and pH. Swim only when that new reading falls within the range your chlorine product’s label allows. For context, the CDC’s pool guidelines describe a typical chlorine range of 1 to 4 ppm and tell owners to follow the manufacturer’s instructions for their product. If your product label gives no range, use that typical CDC range of 1 to 4 ppm as a reference point. If the kit reads zero, do not swim until you have a valid reading within that range, because a very high level can bleach the reagent and read as zero.

Timing matters on ordinary days too. A CDC report on pool chemical injuries estimated the emergency department visits for pool chemical injuries over a three-year period and found that at least 56.3% of those injuries occurred at a residence, and the injury descriptions it reviewed included chemicals added to the water just before a person got in. Add chemicals when nobody plans to swim, and do not plan a swim as a way to use up extra chlorine.

Why Chlorine Ends Up Too High

A high reading usually traces back to something you can change. Common patterns in backyard pools include:

• A dose based on the wrong pool volume. Label doses are set per number of gallons. If the volume you used is higher than the pool really holds, every dose will run high.
• Too many chlorine tablets in a floater or feeder. A floater with its vents wide open, or a feeder dial left at a summer setting, can keep adding chlorine after demand drops. Follow the tablet label and the feeder manual for how many tablets to load.
• Shock added close to swim time. Product labels give a waiting period after a higher dose. Shocking in the evening, as the label allows, gives the level time to come down before the next swim.
• A salt system left on a high setting. Output that suited a hot week of heavy use may be too much for a cool, quiet week.
• A cover left on. Less sun on the water means one of the things that reduces chlorine is held back.

A simple log of readings, doses, weather, and swim days makes these patterns easy to spot after a couple of weeks.

Storing and Handling Pool Chemicals

The CDC’s pool chemical safety guidance, written for pool operators, calls for storing chemicals in original, manufacturer-labeled containers, storing each pool chemical separately in a dedicated location, keeping incompatible chemicals away from each other, protecting stored chemicals from getting wet, and storing them below 95°F and in conditions recommended by the manufacturer. It also says to secure the chemical storage area to limit access, especially to children and animals. The same habits scale down to a backyard shelf.

The CDC’s guidelines for keeping your pool safe and healthy add three habits: read and follow all directions on product labels, wear safety equipment such as masks, gloves, and goggles when handling chemicals, and keep chemicals secure and away from children and pets.

A neutralizer belongs on that shelf plan too. The HASA safety data sheet cited above lists acids and oxidizing agents as incompatible and calls for storage in a cool, dry, well-ventilated area, so give it its own spot with the lid closed tight rather than setting it next to acid or other pool chemicals.

When to Call a Pro

Call a pool service company, or take a water sample to a pool supply store for analysis, if readings stay high after several days with no chlorine added, if your DPD kit and strips disagree, or if a salt system does not respond to lower settings. If a chemical spills, products are mixed by mistake, or fumes build up, move away to fresh air and follow the product label’s directions. The first aid section of the EPA tablet label says that if the product is inhaled, move the person to fresh air, and if the person is not breathing, call 911 or an ambulance. For salt systems and automatic feeders, contact the equipment maker’s support line before replacing parts or touching anything electrical.

More water chemistry guides are in the water care section.

FAQ

How Do You Reduce Chlorine in a Swimming Pool Quickly?

A chlorine neutralizer used at the label dose can bring the level down faster than waiting, and replacing part of the water with fresh water is the other faster option; check local regulations before you drain. Both still call for a new reading before anyone swims. If there is no rush, stopping chlorine additions and letting sun and time work adds nothing new to the water.

Will Pool Chlorine Go Down on Its Own?

Usually, if nothing is still adding chlorine. The CDC lists sunlight and the pee, poop, sweat, and dirt from swimmers’ bodies among the things that reduce chlorine levels. A tablet feeder or salt cell that is still running keeps adding chlorine, so stop those first. How fast it drops depends on sun, stabilizer, and how much the pool is used, so measure rather than guess.

Does a Strong Chlorine Smell Mean There Is Too Much Chlorine?

Not necessarily. The CDC describes chloramines, a type of combined chlorine, as forming in water and off-gassing into the air, and lists the distinctive chloramine odor as a warning sign. Measure free and total chlorine before deciding. If free chlorine is low but total chlorine is high, the fix is to raise free chlorine, not lower it.

Why Does My Kit Read Zero Right After I Shocked the Pool?

The CDC notes that when the water sample holds more than 10 ppm of chlorine, the reading can partially or totally bleach out and show a false low or 0 ppm result. Do not add more chlorine based on that reading. Keep the pump running, following your pump manual’s electrical warnings, and measure again later.

Can I Use Sodium Thiosulfate to Lower Pool Chlorine?

Sodium thiosulfate is the ingredient in at least some pool chlorine neutralizers. Use only a product labeled for pools, follow that label’s dose and handling directions, and add it by itself. HASA’s safety data sheet for its neutralizer lists eye and skin irritation and harm if inhaled as hazards, so gloves and eye protection are part of the job.

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