A refugium sump is a compartment within a marine or reef aquarium filtration system that contains a separate area for growing marine macro-algae and other organisms.

The idea behind a refugium sump is to create a habitat that supports beneficial organisms that can help to maintain the overall health of the aquarium by consuming excess nutrients and competing with harmful algae. The water in the refugium sump is typically connected to the main tank and flows through the refugium compartment before returning to the main tank. This allows for a continuous exchange of organisms and nutrients between the two areas.