Stabilised or unstabilised chlorine: which to use

The difference between stabilised chlorine (dichlor, trichlor and tablets) and unstabilised chlorine (calcium hypochlorite and liquid), and when to use each.

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Every pool chlorine does the same job: it disinfects the water. The difference is what comes with it.

Stabilised chlorine

This is dichlor ("stabilised" granules) and trichlor (most tablets). They already contain stabiliser (cyanuric acid), which protects chlorine from the sun.

• Advantage: chlorine lasts longer in an outdoor pool.
• Watch out: stabiliser doesn't evaporate and isn't filtered out. Every dose adds a little more to the water.
• Above 50 ppm of stabiliser, chlorine loses strength. Well above that, the only fix is to replace some of the water.
• Trichlor tablets are acidic and lower the pH over time.

Unstabilised chlorine

This is calcium hypochlorite (regular granules), liquid chlorine and household bleach. They don't build up stabiliser.

• Advantage: you can use it every day without anything building up in the water.
• Watch out: outdoors, with no stabiliser in the water, the sun destroys chlorine within hours.
• Liquid chlorine and bleach lose strength quickly once opened and tend to raise the pH.

Which to choose

• Outdoor pool: keep stabiliser between 30 and 50 ppm. Many people add stabiliser once and then use unstabilised chlorine day to day, testing the stabiliser every month.
• Indoor or covered pool: the sun isn't a problem, so unstabilised chlorine is enough.
• A tablet in a floater is handy for maintenance, but test the stabiliser and pH often.
• Salt chlorinator: it makes unstabilised chlorine, so outdoor pools still need stabiliser in the water.

Never mix different types of chlorine, dry or in the same bucket. Dichlor or trichlor with calcium hypochlorite can react violently and catch fire.