Refugium
The refugium is a section of the sump where macroalgae (typically Chaetomorpha) grow with dedicated lighting. It is used to export nitrates and phosphates naturally.
Detailed explanation
The refugium is a natural biological filter. The macroalgae within it grow by consuming nitrates, phosphates and other nutrients from the water. When the algal mass becomes too much you remove part of it and with it physically export the nutrients from the system.
Chaetomorpha (chaeto) is the most used macroalgae. It grows quickly, does not become invasive and is easily harvested. The alternative is Caulerpa but it can go into the sexual phase releasing all the spores into the water (a disaster).
The lighting of the refugium should be on the reverse cycle compared to the main aquarium: light in the refugium at night, dark during the day. This stabilises the pH in the 24 hours because when the corals in the main aquarium stop photosynthesising at night (and the pH would tend to drop), the algae in the refugium continue to consume CO2 keeping the pH up.
The refugium also acts as a natural nursery for copepods and amphipods: small crustaceans which then end up in the main aquarium as live food for fish and corals.
In freshwater aquariums the concept of refugium is less widespread. A compartment with fast-growing plants (deep-rooted Pothos, Pistia) has the same effect.
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