Refugium (fishkeeping)

In fishkeeping, a refugium is an appendage to a marine, brackish, or freshwater fish tank that shares the same water supply. It is a separate sump, connected to the main show tank. It is a "refugium" in the sense that it permits organisms to be maintained that would not survive in the main system, whether food animals, anaerobic denitrifying bacteria, or photosynthesizers. For some applications water flow is limited in order to protect plants or animals that require slow flow. The refugium light cycle can be operated opposite to the main tank, in order to keep total system pH more stable (due to the uptake of acid-forming CO₂ by photosynthesis occurring in the refugium during its "daylight" hours). One volume guideline for a refugium is 1:10 main tank volume.

A refugium may be used for one or more purposes such as denitrification, nutrient export, plankton production, circulation, surface agitation to improve oxygen exchange with the atmosphere or even aesthetic purposes.

Tropical marine refugiums often contain live rock and live sand, macroalgae, and sometimes scavenger microfauna such as micro brittle stars, tiny sea stars such as Asterina, snails, and worms. To achieve maximum biological filtration through uptake of nutrients by macroalgae/mangroves, strong lighting is often utilized in the refugium. In this fashion, macroalgae algae growth is encouraged, competing with nuisance algae which would otherwise thrive in the main/display tank. Some also use refugiums to raise tiny brine shrimp, mysis shrimp, ostracods, amphipods, copepods and isopods for delicate fish like seahorses and dragonets to feed on. A sump is an additional tank often kept under the display tank that serves to house filtration and other miscellaneous equipment such as protein skimmers, reaction chambers, hoses, heaters and the like. This is especially common for show tanks and reef tanks. In addition to housing critical equipment, the sump serves to add volume to the total system; mitigating the effects of nutrient buildup or unintentional introduction of foreign substances.

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