Why is my pool pH high?

What a high pH means

pH measures how acidic or alkaline your water is. For pools and hot tubs the ideal is 7.2 to 7.8, and most people aim for about 7.4 - close to the pH of our eyes, so it is comfortable to swim in.

Above 7.8, chlorine becomes much less effective. At pH 8 only around a fifth of your chlorine is in its fast-acting form, so the water can turn cloudy or green even though the chlorine reading looks fine. High pH also leads to scale on tiles and heaters, cloudy water and itchy skin.

Common causes

pH naturally drifts upwards in most pools. The usual culprits are:

• High total alkalinity, which pushes pH up and holds it there.
• Aeration - waterfalls, fountains, spa jets and air blowers all make pH rise.
• Hard, alkaline tap water, common in the south and east of England.
• Some chlorine products, such as liquid chlorine (sodium hypochlorite) and calcium hypochlorite, which are alkaline.
• A newly rendered or plastered pool, which releases alkaline material for several weeks.

How to lower pH, step by step

Always check total alkalinity first. If alkalinity is also above 120 ppm, the good news is that pH reducer lowers both - see our alkalinity guide for the full method.

• Make sure the pump is running and leave it running throughout.
• Work out the dose from the product label or the app's treatment plan, based on your water volume. Start with half the dose - it is easy to overshoot.
• Add the pH reducer to a bucket of pool water (never water to chemical), stir to dissolve, then pour it slowly in front of a return jet in the deep end.
• Never mix pH reducer with chlorine or any other chemical, and never add them at the same time. Wait at least an hour between different chemicals.
• Leave it to circulate - about 4-6 hours for a pool, or 20-30 minutes with the jets on for a hot tub.
• Retest, and add the rest of the dose if pH is still above 7.6.

Keeping pH steady

Test two or three times a week so small drifts can be corrected with small doses. Keep total alkalinity between 80 and 120 ppm, which acts as a buffer. If you have lots of water features, expect to add pH reducer more often, and consider turning them off when not needed.

When to call a professional

If pH keeps climbing within a day or two of correcting it, or you see heavy scale building up on tiles or in the heater, ask a pool professional to check your water balance and equipment.

Frequently asked questions