REFUGIUM

A refugium is an appendage to a marine, brackish, or freshwater fish tank in which water flows in, baffles slow it down to allow a certain amount of stagnation, and flows back out into the tank. Sizes vary from as small as a shoe box to as large as an entire aquarium. Inside the semi-stagnant portion, various suppositories can be placed to allow biological filtration to take place. The items depend on the type of tank, but common items include crabs, snails, or other creatures that will break down solid waste, or certain algae or live plant species to reduce nitrate and phosphate levels. Using natural creatures is more conducive to life in the aquarium than adding chemicals, since many chemical additives produce side effects.
Refugiums often contain live rock and live sand, macroalgae, and scavenger microfauna such as micro brittle stars, tiny sea stars such as asterina, snails, and worms. To get the maximum biological filtration (through uptake of nutrients by macroalgae/mangroves), strong lighting can be used on the refugium. The refugium light cycle can be operated opposite to the main tank, in order to keep the total system pH more stable (due to the uptake of acid-forming CO2 by the refugium during its daylight hours). Some people also use refugiums to raise tiny brine or mysis shrimp for delicate fish like seahorses and dragonets. A sump is a refugium that also contains the main aquarium's other equipment, to keep all hoses, filters, and heaters out of view. This is especially common for show tanks and reef tanks.
In home aquariums, the refugium is typically located in the cabinet underneath the main tank. By confining the plants and creatures to the refugium, the main tank remains clean and aesthetically pleasing, while the biological filtration takes place in the refugium.