Author: Dr. L.E. Freese

What is Chlorine?   
Properties of chlorine?
How is chlorine made? 
What are the uses of chlorine?
Chlorine and water treatment?
Does chlorine affect my fish when I use tapwater for water changes?
How can I remove the chlorine in tapwater?






What is chlorine?

Chlorine is one of 90 elements that are found in nature.

A Swedish pharmacist Carl Wilhelm Scheele was the first to discover chlorine in 1774. He was not aware of what he had discovered, as he thought that the greenish-yellow gas that was formed when hydrochloric acid was poured onto manganese dioxide was a compound of oxygen.

This belief was held as true until 1810 when Sir Humphrey Davy an English chemist realized that the gas was an element and suggested the name " chloric gas" or "chlorine".

Properties of chlorine

Chlorine is a halogen as are iodine, fluorine and bromine. They are highly reactive and are normally found in a compound with other elements such as sodium, potassium and magnesium of which the most commonly known is sodium chloride:- our table salt.

How is chlorine made?

The most common way to make chlorine is to pass electricity through a saltwater solution and this solution will separate into chlorine, sodium hydroxide (Caustic soda) and hydrogen.

2NaCl + 2H2O à Cl+ 2NaOH + H

Salt + Water à Chlorine + Caustic soda + Hydrogen

The gas is dried, chilled and pressurised, or converted to liquid for storage.

What are the uses of chlorine?

Some of the uses of chlorine can be summarized as follows

  • Strong disinfectant e.g. Water treatment plants
  • Pesticides
  • Production of other chemicals e.g. Pharmaceutical industry
  • Production of solvents for metalworking, dry cleaning, and electronics
  • Bleach in pulp and paper manufacture
  • Manufacture of inks and dyes, glues, cements, plastics and soaps

As can be seen chlorine is interwoven into our daily lives.

Chlorine and water treatment?

Chlorine has been added to municipal water for a very long time and this is due to its disinfectant property. When chlorine is added to water it kills bacteria, viruses and other microorganisms that cause disease, by forming a bonds with and destroying the outer surfaces of bacteria and viruses.

There are a few problems with the use of chlorine and the most obvious being the characteristic taste and odour of the water treated with chlorine. The other problem was discovered in the mid-1970’s when it was found that when chlorine was added to water it would react with whatever was in the water and this was good in terms of bacteria and viruses. However when chlorine reacts with organic matter such as decayed leaves a byproduct known as trihalomethanes (THM) is formed, the problem with THMs (e.g. chloroform) is that they may be carcinogenic (cancer forming).

Does chlorine affect my fish when I use tapwater for water changes?

Chlorinated tapwater is safe for human consumption, but highly toxic for fish.

Chlorine is usually added to Municipal water at a concentration of 0.5 to 3.0 ppm (parts per million). Although this seems to be such a small amount, fish are very sensitive to chlorine and concentrations of as little as 0.05ppm are deadly. Larger fish are far more sensitive than smaller fish and are affected by smaller concentrations.

Chlorine not only affects the gills of the fish but it also kills the nitrifying bacteria in the biofilter. The resultant spikes of ammonia and nitrite together with the initial damage done to the fish results in the death of the fish 3-5 days after introducing chlorine into the system.

How can I remove the chlorine in tapwater?

Chlorine dissipates into air relatively easy, as it is a gas dissolved in water. By simply spraying the tapwater into the pond about half of the chlorine will have dissipated into the air. Therefore never stick the end of the hose into the water when adding chlorinated water to a pond or aquarium. The problem with this is that you are still left with chlorine in your water.

Another way is to leave the chlorinated water in a container (preferably aerated) in the sunlight for about 24 hours and the water would be safe for aquarium use. The problem with this is that with the water exposed to the elements unwanted bacteria can enter into the water or unwanted chemicals such as fungicides or pesticides can contaminate it.

Anti-Chlorine binds with the chlorine in the water and makes the water safe for immediate use. Anti-Chlorine can be added before or during introduction of chlorinated water to your aquarium or pond.