i might get a 40 gallon saltwater tank whatis a refugium.

A refugium is essentially another (usually smaller) tank that is connected to the main tank in such a way that water circulates between them.
It can be under the main tank, above it, beside it, hung on the back, or even inside (but divided from) the main tank.

A refugium can be a handy place for various filtration devices, for breeding live food items such as copepods, for keeping baby fish where bigger fish won't get them, for extra live rock, for culturing macroalgae (if the refugium is lighted), for keeping marine mud and biomedia, for installing a skimmer, ozonator, UV sterilizer, or assorted other equipment, and so on. It's sort of like a garage, basement, or attic for your aquarium.

A refugium is a tank, usually smaller and above or higher than the main tank. Water from the sump, instead of going directly to the main tank is pumped into the sump and then flows by gravity into the main tank. The refugium usually contains a sand bed and live marine plants such as Chaetomorpha, a fast growing plant that absorbs nitrates and other unwanted organic wastes. The sand bed and the plants provide a home for small invertebrates and zooplankton where there are no fish to eat them. The word refugium is derived from "refuge". For the inverts and plankton, that is exactly what it is. As long as they stay there! Once they fully populate the refugium, the excess population emigrates to the main tank. Since it flows by gravity they make it there without being chopped up by the vanes of a water pump.However now they may end up as live food for the fish and large invertebrates, including live corals and clams. pets question and answers,www.5d2d.com

The refugium may have a reversed photoperiod from the main tank. It is dark when the main tank is lit and vice versa. Since CO2 may build up at night, this may balance out the Oxygen and CO2 levels reducing the swing that happens in an aquarium between light and dark periods.