When discussing water pumps there are two categories submersible and in-line. Aquarium pumps serve multiple purposes within the tank system. Water movement and filtration being the major uses. Current in an aquarium is critical to the health of every organism and also moves the water through the filtration system.
Submersible water pumps function completely underwater usually located in the sump. These pumps pull water through the filter and into any treatment equipment before pushing it back out into the tank. These pumps are generally quieter, but do transfer heat into the tank if the water is not routed through a chiller before leaving the filtration cycle.
In-line pumps are located outside of the water and are hooked up to an inlet and outlet hose. These pumps are air cooled so heat is not transferred to the water, but are louder. These pumps are usually more powerful than submerged pumps. There are two set-ups available for in-line pumps, pressure or free-flow. Pressure setups force water into the filtration canister causing back pressure which should be monitored. Free-flow setups have the pump placed at the end of the filtration cycle to move it on to further treatment or directly back into the tank. This system produces very little back pressure moving the most amount of water with the least amount of energy.