Refugium (fishkeeping)
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In fishkeeping
Fishkeeping
Fishkeeping is a popular hobby concerned with keeping fish in a home aquarium or garden pond. There is also a fishkeeping industry, as a branch of agriculture.-Types of fishkeeping systems:...

, 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 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 by the refugium during its daylight hours). One size guideline 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, oxygenation or aesthetic purposes.

Construction

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
Brine shrimp
Artemia is a genus of aquatic crustaceans known as brine shrimp. Artemia, the only genus in the family Artemiidae, has changed little externally since the Triassic period...

, mysis shrimp, ostracod
Ostracod
Ostracoda is a class of the Crustacea, sometimes known as the seed shrimp because of their appearance. Some 65,000 species have been identified, grouped into several orders....

s, amphipods, copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...

s and isopods for delicate fish like seahorse
Seahorse
Seahorses compose the fish genus Hippocampus within the family Syngnathidae, in order Syngnathiformes. Syngnathidae also includes the pipefishes. "Hippocampus" comes from the Ancient Greek hippos meaning "horse" and kampos meaning “sea monster”.There are nearly 50 species of seahorse...

s and dragonet
Dragonet
Dragonets are small, perciform, marine fish of the diverse family Callionymidae . Found mainly in the tropical waters of the western Indo-Pacific, the family contains approximately 186 species in 18 genera. The Draconettidae may be considered a sister family, whose members are very much alike,...

s 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.

Tropical Freshwater Refugiums often contain large amounts of aquarium plants, primarily Vallisneria
Vallisneria
Vallisneria is a genus of freshwater aquatic plant, commonly called eelgrass, tape grass or vallis. The genus has 6-10 species that are widely distributed, but do not grow in colder regions....

, Anubias
Anubias
Anubias is a genus of aquatic and semi-aquatic flowering plants in the family Araceae, native to tropical central and western Africa. They primarily grow in rivers and streams, but can also be found in marshes. They are characterized by broad, thick, dark leaves that come in many different forms....

, Cabomba
Cabomba
Cabomba is an aquatic plant genus, one of two belonging to the family Cabombaceae. It has divided submerged leaves in the shape of a fan and is much favoured by aquarists as an ornamental and oxygenating plant for fishtanks...

, Echinodorus
Echinodorus
Echinodorus is a genus of plants in the family Alismataceae and distributed in the Western Hemisphere. Its scientific name is derived from Ancient Greek echius – rough husk, and doros – leathern bottle, alluding to ovaries, which in some species are armed with persistent styles,...

, Microsorum
Microsorum
Microsorum is a genus of over fifty species of tropical ferns. Like most ferns, they grow from rhizomes, rather than roots. The genus name is often mis-spelled "Microsorium" or "Microsoreum". It includes some species that are lithophytic rheophytes....

, Cladophora
Cladophora
Cladophora is a genus of reticulated filamentous Ulvophyceae . The genus Cladophora contains many species that are very hard to tell apart and classify, mainly because of the great variation in their appearances, which is affected by habitat, age and environmental conditions...

, Java moss
Java moss
Java Moss is a moss belonging to the Hypnaceae family. Native to Southeast Asia, it is commonly used in freshwater aquariums. It attaches to rocks, roots, and driftwood...

 and Fontinalis
Fontinalis
Fontinalis is a genus of submerged aquatic mosses belonging to the subclass Bryidae. They are also called Fountain Mosses, Brook Mosses, or Water Mosses....

, but like marine tanks they often contain scavengers such as several species of prolific snails which are often Malaysian trumpet snails, Ramshorn snails
Planorbidae
Planorbidae, common name the ramshorn snails or ram's horn snails, is a family of air-breathing freshwater snails, aquatic pulmonate gastropod mollusks....

 and Physa
Physa
Physa is a genus of small, left-handed or sinistral, air-breathing freshwater snails, aquatic pulmonate gastropod mollusks in the family Physidae.These snails eat algae, diatoms and detritus.- Anatomy :...

 and prolific shrimp such as cherry shrimp. Some use refugiums to raise microfauna Daphnia
Daphnia
Daphnia are small, planktonic crustaceans, between 0.2 and 5 mm in length. Daphnia are members of the order Cladocera, and are one of the several small aquatic crustaceans commonly called water fleas because of their saltatory swimming style...

, Moina
Moina
Moina is a genus of crustaceans within the family Moinidae. The genus was first described by W. Baird in 1850. This genus demonstrates the ability to survive in waters containing high salinity and other impurities, including salt pans...

 and Gammarus
Gammarus
Gammarus is an amphipod crustacean genus in the family Gammaridae. It contains more than 200 described species, making it one of the most speciose genera of crustaceans...

to supplement the diets of the fish in the main tank, particularly fish that are picky about their diet, such as freshwater pipefish.

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.
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