Why is my pool pH low?

What a low pH means

The ideal pH for pools and hot tubs is 7.2 to 7.8, with 7.4 a good target. Below 7.2 the water is acidic. It can sting eyes, dry out skin and hair, and slowly corrode ladders, heat exchangers, pump seals and hot tub heaters.

Acidic water also eats away at grout and plaster and can make vinyl liners stretch and wrinkle. Chlorine gets used up faster too, so you spend more on sanitiser.

Common causes

• Low total alkalinity, so pH has nothing to hold it steady and swings downwards.
• Heavy rain - British rain is slightly acidic and dilutes your alkalinity.
• Trichlor chlorine tablets, which are acidic and gradually pull pH down.
• Too much pH reducer added in one go.
• Soft tap water, common in Scotland, Wales, the North West and parts of the South West.

How to raise pH, step by step

Check total alkalinity first. If it is below 80 ppm, raise the alkalinity before adjusting pH - alkalinity increaser will often lift the pH on its own, and the pH will then stay where you put it.

• Keep the pump running throughout.
• If alkalinity is low, add alkalinity increaser first (see our alkalinity guide), wait 6 hours, then retest pH.
• If alkalinity is fine but pH is still low, work out the pH increaser dose from the label or the app's plan. Start with half.
• Dissolve it in a bucket of pool water - always add chemical to water, never the other way round - and pour slowly in front of a return jet.
• Never mix it with chlorine or other chemicals. Leave at least an hour between different products.
• Circulate for 4-6 hours (20-30 minutes with jets on in a hot tub), then retest before adding more.

Preventing low pH

Keep total alkalinity in the 80-120 ppm range and test after heavy rain. If you use trichlor tablets, check pH more often, as they steadily lower it. Add pH reducer in small doses so you do not overshoot.

When to call a professional

If you notice green or blue staining, metallic taste, or pitting on metal fittings, the water may have been acidic for a while and started dissolving metal. A pool professional can test for copper and iron and advise before you shock or rebalance.

Frequently asked questions