The whole reason for using RO/DI or distilled is stability. You get the same consistent high quality water every time.
With tap water, regardless if its a domestic well, municipal well, surface water or whatever, it can and does change. You have no control over how it comes out of the tap. Even the best treatment plant in the world cannot prevent a contractor from digging up a water main and causing a backflow. The best designed well in the world still changes as ground water levels change or pumping rates change or in rainy seasons and herbicides, pesticides and fuel products wash into the water table.
We as aquarists should be providing the best possible environment for our inhabitants and livestock period. If we are not doing that we have no business in this hobby. We agonize over the best lighting, the best pump, the best live rock or corals but you know what, none of that matters if you don't have good water.
Water is the single largest ingredient in an aquarium, reef or otherwise, and everything in that tank depends on its quality and stability. I don't think it can be put any simpler!
Edited by AZDesertRat, 23 February 2012 - 05:46 AM.
16G Bowfront Nanoreef. 100G SPS dominated reef.