A refugium is a compartment isolated from the rest of the aquarium population that may either be inside or outside of the fish tank. The purpose of the refugium, as its name would suggest, is to provide a refuge for fish and other aquatic life. The refugium, however, shares the same water conditions as the rest of the aquarium, making it an ideal feature if the goal is to eventually release the isolated species into the main tank.
A refugium may be beneficial if a species of aquatic life, such as a fish, is injured and may need special protection from aggressive species in the aquarium. Often, an injured fish is a prime target for attack. The best way to keep these fish safe while they recover is to keep it isolated. For some aquarium owners, this is very important, as some exotic fish species can be quite expensive.
A refugium can also be used to keep prey isolated until they are ready to be presented to the fish population for consumption. This allows the prey to get acclimated to the water; some fish that skip this acclimation process are shocked when they enter the water and die prematurely. This may not sound like a problem since the prey will be eaten anyway, but some fish species specifically adapted for predation will not eat prey species that are already dead. Thus, in some cases, a refugium can be very important for the overall health of the aquarium.
Refugiums are different than most other aquarium dividers and separators. In the common vernacular however, some use the term to refer to any divider or separator included in an aquarium. Still, most refugiums are considered so only if they have a separate compartment located outside of the main aquarium.
In addition to aquarium applications, refugiums can also be used in aquaculture. In such situations, all refugium units will share the same water flow, but each refugium may do something a little differently with that water. For example, in some cases, certain species may need a slower current. Other species may need faster current. Refugiums can help regulate this. It should be noted that refugiums are often used for plant life as well as animal life.