Can I use regular tap water in my grow?
An old outdoor myth to using tap water; is to store the water in a barrel over night, or few days in advance of watering. This will allow the calcium and trace elements to evaporate.
Like Ryan said I do not think it would be just a great idea to use tap water especially if your wanting to do a hydroponic grow. You will need your water to be PH balance so you can work with it as far as adding nutrients and what not.
Depending on your geological location, your tap water may contain trace elements such as calcium and other minerals. Professional growers use sediment filters to remove these elements and bring the Parts Per Million (PPM) of the nutrient mix into a workable range. Depending on the nutrients chosen and your grow system, it may also be wise to include a Reverse Osmosis (R/O) system as well.