What is pool stabiliser (cyanuric acid) and how much do I need?

What stabiliser does

Stabiliser, or cyanuric acid (CYA), acts like sun cream for your chlorine. Without it, sunlight can destroy most of the free chlorine in an outdoor pool within a few hours - even on a cloudy British day.

The ideal level is 30 to 50 ppm. Below 30, chlorine gets burned off quickly. Above about 100, stabiliser starts to slow chlorine down so much that algae can grow even with a normal chlorine reading, sometimes called "chlorine lock".

Do I need stabiliser?

• Outdoor pools: yes, keep it at 30-50 ppm.
• Indoor pools: usually not needed, as there is little UV light.
• Hot tubs: not normally added - covered tubs get little sunlight. Stabilised chlorine granules add a little over time, which is why regular draining and refilling matters.

Why stabiliser creeps up

Stabilised chlorine granules (dichlor) and tablets (trichlor) contain stabiliser. It does not evaporate or get used up, so every dose adds a little more. Over a long season the level can climb well above 50 ppm, and only draining away some water brings it down.

How to raise stabiliser, step by step

• Work out the dose with the app's plan or the label. As a guide, 1 g per 1,000 litres raises CYA by about 1 ppm - so taking a 40,000-litre pool from 10 to 40 ppm needs about 1.2 kg.
• With the pump running, add it slowly. Stabiliser dissolves very slowly, so either dissolve it in a bucket of warm water (chemical to water) or place it in the skimmer basket in a mesh bag, following the label.
• Never mix it with other chemicals in the same bucket.
• Do not backwash or clean the filter for 48 hours, or you will waste undissolved product.
• Retest after 2-3 days - it takes a while to show fully on a strip.

If stabiliser is too high

There is no chemical that reliably removes stabiliser. The fix is to replace some water: draining a quarter of the pool and refilling with fresh water lowers CYA by roughly a quarter. Then switch to an unstabilised chlorine for a while and keep testing.

When to call a professional

Before draining a large amount of water from an in-ground pool, get advice - a high water table can lift an empty pool shell or liner. A professional can also confirm a very high CYA reading with a lab test.

Frequently asked questions