Aside from a few exceptions, beer is more than 90% water. Although most people take brewing water for granted, the chemical composition of water plays a distinct role in shaping a beer. However, extract brewers really shouldn’t need to worry about their water composition. If your water tastes fine straight out of the tap, it’s probably good enough for brewing.
In general, unless your tap water:
- smells or tastes funny (chlorine, sulfur, etc.)
- is particularly hard or soft
- has a pH that significantly deviates from 7,
…your tap water is just fine.
If all else fails, buy some bottled drinking water from the grocery store and get on with brewing already. If you absolutely insist on tinkering with your water chemistry, read John Palmer’s advice on water treatment for homebrewers.