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Too much chlorine in a hot tub: how to lower it

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In short

If chlorine reads too high, do not use the tub. Leave the cover off with the bubbles or filter running so chlorine breaks down in daylight, and retest every few hours. For a faster drop, drain part of the water and top up with fresh water, or use a chlorine neutraliser made for spas, following the label. Use the tub again once chlorine is back in the normal range.

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It is easy to overshoot when dosing a small tub. A heavy hand with granules, a dispenser left wide open or a shock dose followed by a top up can all push chlorine above the range on your test strip. Too much chlorine can irritate and smell strong, and it wears the cover and liner faster. This guide explains how to bring the level down safely and how to avoid it happening again.

Check the reading first

Make sure the strip or kit is in date and that you read it at the time given on the pack. Old strips and late readings can show darker colours than the real level. Our guide on reading test strips covers the details.

Note both free chlorine and, if your kit shows it, total chlorine. Shock treatment can make the reading high for a while, which is expected; the label tells you how long to wait.

pH

Target band: 7.2 to 7.6 (scale 6.8 to 8.0)

too low: pH increasertoo high: pH reducer

Total alkalinity

Target band: 80 to 120 ppm (scale 40 to 200 ppm)

too low: alkalinity increasertoo high: pH reducer

Free chlorine or bromine

Target band: as on the product label (scale 0 to 10 ppm)

too low: add sanitisertoo high: cover off, wait

Common guidance only. Always follow the ranges and safety instructions on your own test strips and chemicals.

Keep each reading inside its target band. Correct alkalinity first, then pH, then sanitiser.

Ways to lower chlorine

MethodSpeedNotes
Cover off, filter or bubbles onSlow, hoursSunlight and air break down chlorine
Stop adding and close the dispenserSlowLets the level fall naturally
Partial drain and refillFastDilutes chlorine and other chemicals too
Spa chlorine neutraliserFastFollow the label carefully to avoid going too low
Options to reduce chlorine

Step by step

  1. 1

    Stop using the tub and take out any floating dispenser.

  2. 2

    Remove the cover and run the filter or bubbles to move the water.

  3. 3

    Leave it open in daylight if you can, keeping children and pets away.

  4. 4

    Retest after a few hours.

  5. 5

    If it is still well above range, drain part of the water and top up with fresh water.

  6. 6

    Rebalance pH and alkalinity after any top up.

  7. 7

    Use the tub once free chlorine is back within the normal range.

Note:Keep children and pets away from an open tub. Never leave an uncovered hot tub unattended if children are around.

Using a chlorine neutraliser

Spa chlorine neutralisers, often based on sodium thiosulfate, lower chlorine quickly. They are effective, which is the risk: too much and you remove all the sanitiser. Measure carefully, add a small dose, let it circulate and retest. Then rebuild a normal level if needed.

Neutraliser is also useful before draining heavily chlorinated water, if local rules or the maker suggest it.

How to avoid overdosing

  • Work out your water volume in litres with our water volume calculator.
  • Dose from the label for your volume, not by eye.
  • Add small amounts and retest, rather than one large dose.
  • Check dispenser vents and take the dispenser out if levels keep climbing.
  • Do not add chlorine and shock on the same day unless the label says so.
  • See our dosing guide for a worked example.

Effects on the tub

Very high chlorine fades and weakens PVC, liners and cover undersides over time, and can make the bladder or lid wear faster. It can also bleach swimwear. Keeping chlorine in range is easier on your tub as well as more comfortable.

Tubs with good filters and covers make it easier to hold a steady level. See our best inflatable hot tubs guide.

See the best inflatable hot tubs →

Why chlorine creeps up

Chlorine usually ends up too high for one of a few reasons. The dose was worked out for a bigger tub than yours, often because the volume was guessed. A floating dispenser was left with its vents wide open while the tub sat unused, so tablets kept dissolving with no bathers using the chlorine up. Or a shock dose and a normal dose were added close together.

Cool weather and a closed cover also slow how fast chlorine is used. A dose that suited a busy summer weekend can be too much in a quiet week in autumn. Adjust your routine to how often the tub is used, and test before dosing rather than adding chemicals on a fixed schedule.

Keeping a short log of tests and doses makes patterns obvious. After a few weeks you will know roughly how much your tub needs in a typical week.

Draining part of the water

A partial drain is the quickest reliable fix when chlorine is far too high. Switch off and unplug the pump unit, drain a portion of the water with a hose from the drain valve, then top back up with fresh water. Reconnect, run the filter and retest. Our draining guide explains where to send the water.

Frequently asked questions

How long does it take for chlorine to go down in a hot tub? With the cover off and the filter running, it often falls over several hours to a day. A partial drain or neutraliser is faster.

Is it safe to use a hot tub with high chlorine? Wait until free chlorine is back in the normal range on your test before using it.

Will a strong chlorine smell mean too much chlorine? Not always. A strong smell can also come from combined chlorine. Test the water and shock if the label suggests it.

Does heat lower chlorine? Warm water and use do use up chlorine, but the cover off and fresh air are what speed it up.

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