Stabilized or unstabilized chlorine: which to use

The difference between stabilized chlorine (dichlor, trichlor and tablets) and unstabilized 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.

Stabilized chlorine

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

• Advantage: chlorine lasts longer in an outdoor pool.
• Watch out: stabilizer doesn't evaporate and isn't filtered out. Every dose adds a little more to the water.
• Above 50 ppm of stabilizer, 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.

Unstabilized chlorine

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

• Advantage: you can use it every day without anything building up in the water.
• Watch out: outdoors, with no stabilizer 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 stabilizer between 30 and 50 ppm. Many people add stabilizer once and then use unstabilized chlorine day to day, testing the stabilizer every month.
• Indoor or covered pool: the sun isn't a problem, so unstabilized chlorine is enough.
• A tablet in a floater is handy for maintenance, but test the stabilizer and pH often.
• Salt water pool: the salt cell makes unstabilized chlorine, so outdoor pools still need stabilizer 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.