Fish live in oceans, rivers, and bodies of water that have natural currents which constantly distribute food, nutrients, and oxygen. All aquariums benefit from flow by helping the inhabitants stay healthy and stimulated.

Laminar flow can be achieved by placing the filtration system intake and return at opposite ends of the aquarium. For some freshwater aquariums, this is sufficient. More flow is usually needed for saltwater aquariums since ocean waters are naturally very turbulent. Good flow is paramount for a reef aquarium since coral rely on currents to deliver their food and remove waste.

Wavemakers or powerheads are submersible pumps that are placed at various angles and intensities to generate random water movement inside an aquarium.

A secondary plumbing system can be used with a pump to move water out of and into the aquarium to create flow without passing through the filter. A pump used in this application is called a closed loop pump - usually required for large reef aquariums more than around 200 gallons.

Designing a flow system depends on the requirements of the residents. Choose flow rates that will allow for the aquarium volume to turnover 10 to 30 times per hour and use several pumps to minimize stagnant areas in larger aquariums.

AC (alternating current) is the long established tech. DC (direct current) is more advanced, allowing for smaller, quieter, and more powerful pumps that conserve energy and generate less heat. DC pumps can have controllable flow rates, so GPH is usually a range and one pump will work for many applications. AC pumps are only slightly adjustable, if at all, so the correct size pump must be used for each application.