They hook up to a cold water line. Water goes into a 1, 5, or 10 micron particle filter then it moves into a carbon block filter. After that it moves into the ro chamber. It's really to hard to explain. Then it either comes out or it goes through a di filter. Depends on what filter you get. You should do a search on RO/DI filters from a manufacture. http://www.airwaterice.com/Reefkeeper%20Home%20Page.htm