water uses filters and a RO
membrane that the water goes through. A RO
unit uses filters, RO
membrane and a Deionized unit.
This excerpt from Liveaquaria on the one I use explains it all.
"The 4-stage Maxxima RO
takes the full size RO
to a higher level of filtration. The Maxxima is an excellent selection for reef enthusiasts and other serious aquarists, especially those who live in areas with exceptionally high levels of phosphates in their tap water. Includes: TFC (Thin Film Composite) membrane, carbon and sediment pre-filters, mixed-bed deionizing post-filter, with clear canister housings. The post deionizer takes the small percentage of contaminants that the membrane could not remove and filters this to greater than 99.9% pure."