Unless you have a private well, the chances are that your local municipality adds chlorine or chloramine to the tap water to make it safe for humans to drink. Unfortunately, both of these substances are toxic to fish and invertebrates.
Chloramine is more often used today than chlorine because it is more stable. A few water departments still use chlorine, but they can change at any time, and they do not have to notify you when the charge is made. Since chloramine is harder to deal with, and the treatment with chloramine will also effectively treat chlorine, we will discuss the treatment of chloramine.
Commercial chloramine neutralizers are widely available, usually in the form of bottled liquid. Look for a product that specifically states that it will neutralize chloramine, not just chlorine. I strongly suggest that you treat all water going into your aquarium unless you are using water from a private well and are certain that no chlorine or chloramine is used.
If the levels of chlorine or chloramine in your tap water are low, the treatment won’t cause any harm. If they are high (and this can happen unexpectedly, especially in the summer), the treatment will save the lives of your fish. In most cases, you can add the chloramine neutralizer directly to the aquarium just before you add the tap water. Some “water treatment” products make claims about doing all sorts of other beneficial things to your tap water. These are not necessary for a healthy tank, and are more marketing hype than anything else. Some can actually be detrimental.
Source: Sunken Gardens/ By: Karen A. Randall