Circulation pumps allow the water to flow in the system, for example, to return the water from the sump to the aquarium, or, create, in the aquarium a constant or turbulent flow.
Water circulation in the system reduces the risk of thermic shock, helps in the water oxygenation, residues removal, food transportation in the water column (as per example phytoplankton), and prevent the accumulation of harmful organisms (that proliferate in stagnated water as is the case with cyanobacteria).
It is recommended to use a pump suitable to the necessary water quantity needed to be moved, and the type of flow necessary (weak, medium or strong) depending on the resident animals of the aquarium.
In its majority, SPS corals prefer a strong flow. These animals need this type of flow in order to obtain food, and also to remove the residues from its polyps. On the other hand, soft and LPS corals are more sensible, and prefer a weak to medium flow. Water movement is essential in saltwater aquariums, as no coral depends only of its photosynthetic organisms to feed itself, and stagnated water is harmful to fish health, as each species will require a specific flow.
The positioning of the pumps is also important, and these should be placed in a way that there is not obtrusion on the flows pathway (where it could be impeded by rocks, corals and/or algae). It is essential to avoid dead spots (area where there is no water flow), where residue will accumulate. One can also try and recreate the random natural currents, with the right positioning of the pump throughout the aquarium.