You can use tap water, but you need to adjust the pH before you give it to the plants, 5.8 pH for hydro, and 6.5 pH for soil. You can buy a pH “Down” additive from a Hydro store or online. Or you can use pure sulfuric acid, you can get this at an auto parts store as a “battery refill”, this is pure sulfuric acid and does not have any lead in it yet as it has not been inside an auto battery yet.
We spoke about this in this other thread awhile ago: Lowering the pH of my water