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Chlorine (Cl) or Free Chlorine is a halogen chemical element.

In its free form when added to water, it's in fact more correctly called dichlorine or Cl2.

As the chloride ion, which is part of common salt and other compounds, it is abundant in nature and necessary to most forms of life, including humans.

It has a disagreeable suffocating odour and is poisonous.

Chlorine is an important chemical for some processes of water purification, in disinfectants, and in bleach as it is a powerful oxidant.

Chlorine is also used widely in the manufacture of many every-day items, or to purify water in various forms.

  • Used (in the form of hypochlorous acid) to kill bacteria and other microbes from drinking water supplies and swimming pools. However, in most non-commercial swimming pools chlorine itself is not used, but rather the mixture sodium hypochloride, a mixture of sodium and chlorine. Even small water supplies are now routinely chlorinated.
It is this use of chlorine in tap water that brings it into contact with the Aquarium owner.
  • Tap water that has chlorine in it has to be treated with a suitable water conditioner bottle to remove or neutralise this toxic chemical before being added to the aquarium otherwise it will kill your animals.

There are numerous water conditioners on the market, but due to the risk of your water supplier switching to using chloramine without notifying you, it is safer for your aquatic animals if you always use a water conditioner which removes chloramine as well.

  • Note there are some commercial bottles that simply split (or break the bonds of) chloramine into ammonia and chlorine, then remove the chlorine. So these leave the ammonia behind! These products are probably best avoided. See water conditioners for a list.

Free Chlorine

Chlorine is added to tap water in the form of a gas by your tape water supplier. This forms wikipedia:hypochlorous acid (HOCl). In alkaline water this turn into wikipedia:hypochlorite (OCl) (in acidic water it forms wikipedia:chloric acid), and is the active form of chlorine and exists as a free ion. Therefore it is called 'free chlorine'.

Combined Chlorine

When free chlorine combines with contaminants in the water, such as oils, ammonia or other organic compounds (organic amines) like fish waste, it becomes combined chlorine, or chloramines. Tap water suppliers will deliberately add ammonia and chlorine into tap water so that it becomes chloramine.[1]

Total Chlorine

This is the measurement of free chlorine and combined chlorine.

Methods that remove Chlorine

Due to chlorine's nature, it is easily removed from tap water by simply letting the water sit in an open bucket for 24 hours and aerating it strongly.

Indeed in the 1970s a lot of Aquarium books at that time would advise that you set up a tank and leave it for 4–5 days without fish to allow the chlorine to disperse. This wouldn't work with chloramine as it take 2 weeks to disappear.

Due to chlorine's far too easy method of removal from tap water, more water suppliers are switching to using chloramine to kill bacteria and microbes in our tap water. So be sure your tap water contains only chlorine if you use this method as your tap water supplier may switch without warning.

  • Adding Sodium thiosulphate (available quite cheaply from chemical shops) to water will remove it.

Products that test for its presence in water


  1. See Forms of Chlorine by Edstrom Industries

External links