RODI water is generally considered a must have for a reef tank. If you have a small tank you can get away with buying RO or RODI water from the lfs or supermarket. A tds meter is a must to test the water you are getting.
Tap a/o well water contain too many minerals, most of which are unwanted in a reef tank. While the levels are safe for human consumption they could cause problems in a reef tank, like high nitrates, PO4, etc. That leads to algae problems that just won't go away.
Several of our sponsors sell the units. I have the Typhoon III from airwaterice which I connect to my kitchen sink when I need to make change or top off water.