Well, my tank is for local marine life, and the water is from the local ocean. The cleanest water source I know of around here is a 30 minute drive to the south at the Scripps Pier (La Jolla). I can only make it down there 2 or 3 times a week before dark due to work so I think it would be wise to always have at least 10 gallons of useable water on hand ready to go in case of a spike, problem, spill, etc. There will already be salt in it, phytoplankon and who knows what else sucked up through the pump at the pier.
-Blesum