A sump/refugium is a smaller aquarium that sits either to the side or below the aquarium. It is important for many uses. First, the sump/refugium allows you to add more water volume to your system. There is an old saying in saltwater that the solution to pollution is dilution. Also, the refugium can be used as a place of 'refuge' for smaller or more docile animals that may otherwise be harrassed in the display tank. In addition, the sump will also allow you to hide equipment such as heaters and skimmers that would otherwise be visible and unattractive in the display. In many cases, different kinds of macro-algae are placed in the refugium. While growing, this macro-algae helps to reduce nitrates and phosphates from the system while also providing hiding places for copepods and brittlestars while they are reproducing.