A tank that sits external to your main tank but shares the same water via an overflow or attached to a sump. Refugiums are generally used to isolate certain plants or grow items that need to be isolated such as macro algae.
A separate tank which shares water and filtration with the main aquarium. Used for growing Nitrate reducing Algae, raising live food, and isolating inhabitants from the main tank.
a body of water physically separated from the main tank, but which shares a common circulation and water supply, for the purpose of growing beneficial animals and plants)