You need a ro/di unit if you have any trace of no3/po4 in your tap water. If not it's not needed but still helpful in providing the purest water possible for your tank. RO only water can still have no3/po4 but ro/di filters out all harmful elements in tap. DI alone isn't recommended either since the resins are used up quicker.
EBAY has a ton to choose from at reasonable prices.
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-Micah-
If you haven't figured it out yet I like to BOLD links :P
Thanks a lot that clears up everything. I live in an area where the water has lots of calcium, not certain of the other stuff , will buy test kits to check it out.