Some folks prefer to do it that way (with tap water) so it's probably ok, especially if you are going to go through a full cycle anyway. When you are ready to get RO/DI there are several local fish stores that sell it by the gallon (salted or otherwise). I know we bought from Aquarium Designs when we were new to the hobby and again recently during an emergency. We also have bought water from that Windmill place in the parking lots of HEBs.