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Using Municipal Tap Water
Local water companies add chemicals to the water to make it safe to drink (e.g., by adding chlorine or chloramine to kill bacteria). More recently, concern about water flowing through (older) lead pipes has caused some water utilities to add pH-raising chemicals to the water (lead dissolves less readily in alkaline water). Consequently, tap water must be specially treated before it can safely be used in fish tanks.
Another potential problem to be aware of concerns variability in the chemical properties of your water supply over time (e.g., month-to-month). Some water districts don’t have enough water themselves, forcing them to purchase additional water from neighboring water districts in times of shortages. If this water has a different hardness (for example), your tap water’s hardness will vary as well. As a common example, high bacteria levels are more of a problem in summer than winter, especially in warmer climates. Consequently, it is not uncommon for water companies to use more chlorine in summer months to keep bacteria in check. Even such factors as local weather can have an impact; heavy rains may cause the hardness of your water supply to decrease, as local reservoirs fill.
In general, chlorine and chloramine are the two additives that cause the most problems. Note that these two substances are VERY DIFFERENT! Be sure you know what is in your tap water and treat appropriately.
Source: users.cs.duke.edu/~narten/faq/chemistry.html