Red Sea ReefCan Reef ReservoirsThe ReefCan 18 and ReefCan 40 can supply the ATO of a 50 gal or 130 gal tank with enough RO water for about a week. They fit in the sump cabinet or behind the tank, can be refilled without having to remove the reservoir and disconnect its tubes & cables, and have a transparent plastic body with an embedded water scale that allows you to see how much RO water is left ReefCan features: Slim & tall design that holds more RO water while taking up less