A sump is an additional tank placed beneath your marine aquarium. It collects drainage and provides space to house essential equipment such as protein skimmers, calcium reactors, and heaters.

While a sump is not strictly necessary—especially if your aquarium has built-in back filtration or you use an external filter—it is highly recommended for optimal marine setups. A sump increases your total water volume, provides extra stability in water parameters, and allows you to house more equipment out of sight, keeping your display tank uncluttered.