A weir is used in aquariums with sump filtration systems located beneath the aquarium. Water in the aquarium flows over a divider in the corner of the aquarium (the weir) and down a pipe into a separate container under the aquarium (the sump) containing filtration equipment. The filtered water is then pumped back into the aquarium. A weir system maintains a constant level of water in the aquarium, whilst the water level in the sump may fluctuate. Some sump/weir systems can contain an automatic top-up system fitted to the sump tank.