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What's In Your Local Water
In addition to the additives described above (chlorine & chloramine), municipal water may (or may not!) contain other elements that the aquarist may need to know about about. Water in some locations actually contains nitrates. In some places, water contains elevated concentrations of phosphates (1 ppm or more). High phosphate has been linked to algae problems, and a comprehensive algae control strategy may require removing phosphates. High levels of iron (1 ppm or more) have also been linked to thread algae. Consult the algae section of this FAQ for more details.
You may also want to find out just what the local water company is adding to the water?
The quick answer is to ask someone who knows. A local fish store (if they reside in the same water district as you do) should be able to tell you. Alternatively, call your local water utility. Ask to speak with the “water chemist”. Tell them you are an aquarist and want to know about the pH, GH, and KH of your water, as well as how much the water characteristics vary from month to month. Finally, (in the US) if you really want details, have them send you a copy of the periodic water report they are required to generate for the EPA. It contains a detailed listing of exactly what your water contains and in what concentrations (e.g., iron, nitrates, phosphates, etc.). By law, the report is available for public inspection.
Source: users.cs.duke.edu/~narten/faq/chemistry.html