you can use tap water with hydro. bottom line. yes r/o water would be better, but if you dont want to invest in an r/o system or lug jugs around like me, you can use tap water just fine. imo and experience the only thing to really watch for is calcium/salt buildup on your emitters and medium. i used tap water and botanicare cns17 through a whole grow with no adverse effects, just crusty topsoil. (however, if you are an aficianado, you might argue that chlorine and deposits could effect the taste of meds, i couldnt tell)
our water may be stored in canals or who knows where but it all goes through a water treatment center before it goes in the pipes, and its all treated with chlorine and chloramines. you can use tap water even if it doesn't sit first, chlorine isn't going to hurt your plant, it may not have grown to its full potential, but you'd never know. most of us never let our plants get big enough to see "full potential" anyway and full potential only exists under true sunlight, so don't worry about itty bitty chlorine.
organic and veganic gardeners stay away from chlorine and chloramines because it kills all the beneficial organisms we need to breakdown the organic matter we use to feed our plants, but in a hydro setup, you have no beneficials to kill i would use r/o if you can just to help your system stay cleaner, plus its nice to know whats in there (or whats not in there). I used r/o water(40ppm) through my last grow because i paid a lot of money for my house and garden lineup (claims to be the cleanest available lineup) and it didn't make sense to me to dirty it up with my tap water(840ppm) r/o water will also need to a cal/mag supplement