Marineman, you haven't listed how big of a tank you have. CaferacerMike had a post where he broke down all the cost of buying and running a ro/di unit and unless you had a 125gal or bigger it doesn't really pay to run your own. I have a 75gal, with water changes and top off I spend about 15/month buying water from RCA. It might pay off in the long run to buy my own unit but it would be a v e r y l o n g r u n. The plus is I get to go see whats up for grabs every week and a half. Over the past couple of years I have built a friendship and rapport with Jake that would not happen if I wasn't there.