Closed Loops
Closed loops are where the water pumps used for moving water about is located externally to the aquarium. Pipework is attached to both the inlet and the outlet of the pump and both the outlet and inlet are submerged underwater either via the pipework itself or drilled holes in the aquarium with bulkhead fittings attached. When the pipe is filled with water (primed) and the pump turned on there is no head loss experienced on the pump as the water does not have to fight against gravity. Benefits of this type of devices are that the physical unit is removed from the aquarium which therefore means that a heat source is removed as well as a large unsightly item or items. It does need to be noted that a closed loop system does need careful planning and is more suited to aquariums which are in the process of being setup. This does not mean that closed loops can't be used on an existing aquarium as they can but they are slightly harder to install.
With all of the devices listed above it is important to note that the correct placement of the device is the secret to its success. The best example I can use for this is the powerhead. If the aquarium in question has a requirement for 1500gph of water movement and three powerheads are purchased to facilitate this requirement then it is not simply a case of installing the powerheads into the aquarium. The powerheads need to be placed in the aquarium so that the flow provided by the powerheads clash with each other, bounce off the glass, bounce off the rocks etc. They could be provided at the rear of the aquarium facing forwards or at slight angles to each other.
The placement of closed loop systems is slightly trickier is you have to attempt to hide the pipework. A single closed loop is powered by one pump however the aquarium can have more than one loop in the aquarium. Each loop, dependent upon the size of the tubing used can have more than one outlet and these outlets can be placed in numerous placed in the aquarium. You could have two in the top from corners of the aquarium, two in the front bottom of the aquarium other outlets hidden in the rockwork. The pipe at the bottom of the aquarium could be hidden by sand etc, the one at the top could be hidden on the aquarium strapping and the others hidden down the rear of the aquarium. The installation of outlets using closed loops is endless - the hard thing is hiding the pipework!
Every aquarium is different due to size, rockwork layout, corals etc. The placement of the device therefore requires careful consideration as well as a bit of fiddling.