How do I raise low alkalinity in my pool?

Why low alkalinity is a problem

Total alkalinity (TA) keeps your pH stable. The ideal range is 80 to 120 ppm. When it drops below 80 ppm, pH swings about with every rain shower or chemical dose - a problem known as "pH bounce".

Low alkalinity usually comes with low pH, which makes water acidic: it stings eyes, corrodes metal and heaters, and can etch plaster and wrinkle vinyl liners.

Common causes

• Soft tap water, common in Scotland, Wales, the North West and parts of Devon and Cornwall.
• Lots of rain diluting the water.
• Regular use of acidic products such as pH reducer or trichlor tablets.
• Draining and refilling, especially a hot tub.

How to raise alkalinity, step by step

Fix alkalinity before you adjust pH. Alkalinity increaser (sodium bicarbonate) raises TA with only a small rise in pH, and often brings a low pH up to where it should be on its own.

• Work out the dose with the app's plan or the product label. As a guide, about 1.7 g per 1,000 litres raises TA by 1 ppm - so raising a 40,000-litre pool by 20 ppm needs around 1.4 kg.
• With the pump running, dissolve the dose in a bucket of pool water (chemical to water, never the reverse). For large doses, split it over several buckets.
• Pour it slowly around the deep end or in front of a return jet. In a hot tub, add it with the jets running.
• Do not add any other chemical at the same time, and never mix products in the same bucket.
• Wait about 6 hours for a pool (1 hour for a hot tub), then retest.
• Once TA is in range, check pH and adjust with pH increaser only if it is still below 7.2.

Keeping it in range

Test after heavy rain and after large top-ups. If you use trichlor tablets or add pH reducer often, check TA weekly. Soft-water areas may need a small top-up of alkalinity increaser every couple of weeks.

When to call a professional

If TA drops quickly again after every correction, you may be losing water through a leak and topping up with soft water. A pool professional can check for leaks and look at your overall balance.

Frequently asked questions