A closed loop is usually a external pump that sucks water from a central location on the tank and the water goes through the pump and back into the tank at several locations helping eliminate powerheads, or lowering the amount needed.
On mine I have a 1-1/2" bulkhead in the middle of the backwall ( a little off center to the right) that is the suction line. The water returns after the pump to the manifold line on the bottom. I have four 1" risers coming off the manifold the go back into the tank, 2 on near top, 2 on the near bottom.
Inside the tank on each is a loc-line Y with a pair of 1/2" fan nozzles creating flow in 8 different directs, all from a single pump.
The 3 lines in the middle are drain lines.