Why do I have to Shock at night?

Why do I need to add my pool shock at night?

Some shocks and sanitizers need to be added at night because they are not protected from the Sun.

The Sun can reduce chlorine by as much as 90-95% in only two hours in shallow water.*  Chlorine and bromine by nature will burn off in the Sun, but some chlorines have Cyanuric Acid added to them to prevent the Sun from burning it off as quickly.  The pro of this is that it can be added anytime, day or night, without worry that it will simply evaporate.  This is especially nice is you wake up to a cloudy pool on the day you are supposed to have a pool party.

The major con of adding Cyanuric Acid is if there is too much in the water it causes the chlorine to become sluggish, so it will not work as well.  If you have an automatic cover and a cartridge filter this is more common, since there is very little water turnover and the cyanuric acid can build up too much over time.

Bromine is not protected by Cyanuric Acid like Chlorine is, so it is mainly used in spas and indoor pools.  Outdoors it simply gets burned off by the Sun too quickly.

Which shocks can be added during the day?

  • BioGuard Smart Shock
  • BioGuard Super Soluble
  • BioGuard Power Chlor
  • Bioguard Oxysheen

Which shocks need to be added at night?

  • Burn Out 35
  • Burn Out 73
  • BioGuard Lithium Hypochlorite
  • Liquid Chlorine


*Pool & Spa News “Is CYA Out on the Inside?”

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