Refugium (fishkeeping)

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In fishkeeping, a refugium is an appendage to a marine, brackish, or freshwater fish tank that shares the same water supply. For some applications water flow is limited in order to protect plants or animals that need very slow water. 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). One size guideline is 1/10th main tank volume.

A refugium may be used for one or more purposes such as denitrification, nutrient export, plankton production, circulation, surface agitation, oxygenation or aesthetic purposes.

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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. 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.