Get to know the ReefCans, the refill reservoirs specially developed for freshwater and saltwater aquariums. 

The ReefCan 18 and 40 can supply the ATO of a 200-liter or 500-liter aquarium with sufficient water for about a week. They fit in the technical tank cabinet or behind the tank, can be refilled without having to remove the tank and disconnect its hoses and cables, and have a transparent plastic housing with an integrated water scale that allows you to see how much osmosis water is left . 

Check it out:

ReefCan features:

Slim and tall design that holds more osmosis water while taking up less space
Separate holes for the ATO pump cables and water refills
Transparent plastic body with recessed water scale