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Found This Useful.

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Hi all,
On another forum we were debating how good dechlorinaters were because some people were saying they are a waste of money and others saying that certain fish are sensitive to dechlorinaters ( some native New Zealand fish) and this is what one person said. I thought I should pass it on as I found it helpful. :)
 
When you add chlorine to water you get hypochlorous acid which reacts with nitrogen compounds like ammonia,urea and the amines (in all proteins) and forms monochloramine. When you add more chlorine you get dichloramine and even more you get trichloramine. All these (and other reactions) form part of the "chlorine demand" in the water. You cannot get free available chlorine until this "chlorine demand" is satisfied. Therefore when the reaction is pushed towards trichloramine there will be virtually no monochloramine present. In some states in the US they treat the water with monochloramine (made by reacting chlorine with ammonia) because chlorine will react with other impurities in the water and form some compounds that are not so nice (such as acetone) where as monochloramine will not. Monochloramine is not as effective in treating water as chlorine which is used in NZ but is still a strong oxidising agent.
When people complain that the chlorine in a swimming pool is too strong and it is burning their eyes the problem usually is that the free available chlorine has been used up by contaminants in the water (such as urea) and this has pushed the chloramines back towards the monochloramine and this is what is burning their eyes. The problem is fixed by adding more chlorine.
When you allow water to stand or aerate it to get rid of the chlorine the chloramines all move back to monochloramine and this will react with your fish the same as an under chlorinated swimming pool will with your eyes. 
Chlorine and all chloramines can be converted to more harmless chemicals with the addition of sodium thiosulphate which is the active ingredient you are buying from the petshop with dechlorinating products.
Drinking water will contain various impurites that add to the chlorine demand and will form chloramines and other compounds. Ammonia is a bi product of the reaction when adding thiosulphate to chlorinated water. Chlorine only is used to sanitize water supplies in NZ and when you add chlorine you will always get chloramines unless you use distilled water.
 
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