The quality of the water you use in a hydroponic system, whether its source is tap, well, or spring makes or breaks a hydroponic operation. A crucial step in determining feasibility of a commercial site for growing with hydroponics is the Water Analysis. It will tell exactly what minerals or elements are dissolved in your water and the pH (how 'hard' or 'soft' the water is.)
While many commercial sites have excellent water sources, others may have a mineral content that requires a modification or custom nutrient formulation that works in harmony with the minerals in your water. Other sites with significant mineral imbalances out of the tap require some level of treatment. You should know that before you invest in a system.
Water treatment comes primarily in the form of either pH adjusters, nutrient additives, carbon filtration, particulate filters or physical treatment like reverse osmosis that removes all of the minerals in a solution, giving you 'perfect' water.