A Refugium is a place were (as the name implies) various animals and critters can live in refuge from predators. here these shrimps and amphipods and other zooplankton will breed and reproduce and eventually some will find their way back to the main aquarium and will be a very welcoming treat for your fish, corals and invertebrates. An other useful way for the refurgium is to put a deep sand bed to help in the nitrifying process of an aquarium.