Red Sea ReefCansThe ReefCan 18 & 40 can supply the ATO of a 200 litre or 500 litre tank with enough water for about a week. They can 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 floor