A refugium is a place were fauna (whether it be livestock or plant) can grow and reproduce without predation. Usually a seperate container placed under, beside, or over the aquarium. According to definition however, you could also have an in the tank refugium. The most popular item grown in a refugium is macroalgea, normally Caulerpa sp.. Macroalgea use nitrates for nourishment, and therefore keep nitrates to very low levels. Refugiums are also used to raise copepods, amphipods, and other reef critters. Refugiums will require lighting and a flow of water through them. They can be as simple as a container with some Caulerpa or as complex as a seperate tank containing plants, invertebrates, and even fish. HTH