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Forum - Hang On Tank Refugium. A refugium is used to grow macro algae and smaller organisms (like amphipods and copepods) for the animals in the main tank. You can also use the refugium as an algae filter. As the macro algae grows it uses nitrates and phosphates and you can crop the algae as needed. The macro algae in the refugium will help keep nuisance algae from growing in the display tank (if the refugium is attached to the main system).