OK, I have decided to get a MIGHTY MITE w/ DI from airwaterice.com. Does anyone how long before I need to change the filters? Since I have a 20G tank, wondering if I should buy extra set of filters, or just get the unit. Thanks everybody for your input. So is this unit a good unit? I saw some in ebay for about $120, and Drs Foster and Smith have units starting around $150. Any recommendations? Finally, what's the difference between RO only unit and RO/DI? Thanks, again.