Bad plumbing can quickly reduce the force of a 3600 gph pump to a mere trickle, especially on larger aquariums. All of the pieces of PVC quickly add up to several feet of water to push and all of the 90 degree angles severely cut flow rates at every turn. Additionally, closed loop setups will not be effective if you have too many outlets. The more outlets you have means each one is getting less flow. Think about it, four outlets running simultaneously off of a 3600gph water pump will each put out a maximum 900gph, assuming they are all of equal distance from the pump. However, if you use a flow alternating device such as an Oceans Motions that kept half outlets closed at all times and the other half open, the amount at each hole would double to 1800gph.