One of the most important requirements of a marine aquarium is water movement for in-tank circulation or to feed filtration equipment. Water pumps fulfil this role.
The term powerhead refers to a pump that is designed to operate submerged within the tank or sump and they typically do not handle back pressure very well (flowrate drops off quickly when there is resistance to water flow). A pump is designed to be operated externally (or sometimes submerged) and can handle back pressure better.
Powerheads tend to be smaller and have to be submerged in water for operation. The water provides cooling for the motor driving the powerhead.
Another important thing to note is that for a submerged powerhead, all the the energy it consumes is transferred to the water, therefore acting like a heater of the same wattage. For example, if the powerhead consumes 100W, then it will add the same amount of heat to the system as a 100W heater.
Tunze Streams (plus also Seio and others) are pumps that use a propeller instead of an impeller to move the water. Due to their design, they provide a much wider stream of water rather than the thin stream that typical powerheads generate. Additionally, for a similar sized powerhead and energy consumption, they can provide significant large flowrate of water. This allows them to create a large amount of water movement over a large area within the tank. They are quite large units and cannot be used as filters. Some can also be controlled electronically for speed and variable modes.
A closed loop is a method of providing water circulation in a tank without having to have the pump within the tank itself. Water is taken from the main tank through an outlet and a pump returns the water back to the aquarium through inlets (typically two). The inlet and outlets of the piping (including the pump) are submerged within the same tank. There are a number of ways of setting them up, but the principle is still the same.
Submerged Versus External
A pump can have the option of being operated either submerged or externally. Which is possible depends on the brand and make of the pump. Typically the larger volume pumps have to be operated externally.
- An Engineering View Of Aquarium System Design: Pumps And Plumbing by Sanjay Joshi, Nathan Paden and Shane Graber - Advanced Aquarist's Online Magazine
- Pump Selection Guide - Reefs.org
- What is Pump "Head-Pressure?"