In fishkeeping, a sump is an accessory tank in which mechanical equipment is kept. A remote sump allows for a clutter-free display tank.
It is found in mainly in a reef system. The sump sits below the main tank and is used as a filter, as well as a holding place of unsightly, miscellaneous equipment such as protein skimmers, calcium reactors, heaters and the like. The main advantage of having a sump plumbed into an aquarium is the increase of water in the system, making it more stable and less prone to fluctuations of pH and salinity, and also mitigating the effects of nutrient buildup or the unintentional introduction of foreign substances. In addition, some sumps have a compartment that can be converted into a refugium, helping to filter out excess nutrients such as nitrates.