This article is from the Beginning Fishkeeping FAQ, by Thomas Narten with numerous contributions by others.
You may have access to well water instead of municipal tap water. One
advantage with well water is that you don't need to deal with chlorine
and chloramine. On the other hand, well water is frequently (much!)
harder than water available through local utilities. In addition, the
only way to know its composition (GH, KH, etc.) is to run tests on it
yourself. Alternatively, there are companies to which you can send
water samples that will perform a detailed analysis of its contents
(for $20-100).
One potential problem with using well water is that it frequently
contains high concentrations of dissolved gases (which may be
dangerous to fish). For example, well water is frequently
supersaturated with CO2, which lowers the water's pH. Once the CO2
escapes, the pH will increase. Fish shouldn't be subjected to this
temporary pH fluctuation. For safety, aerate well water thoroughly for
several hours before adding it to your tank.
 
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