cl
thanks for the pics and comments, the 4 holes that i have,(i think are 11/2" holes), but i am going to take a picture of the backside of the tank so you can see the holes, and here's question, is the cl system more beneficial or just a different setup(i should say what are the pros and cons of the cl), and i noticed no sump on some setups, does it matter to have a sump or not when having the cl setup? thanks in advance. for the pump i was thinking of using a iwaki 40, since i want a sump and that will have a return pump back into the tank.
As for the closed loop, what size holes are they and are they in any way part of the overflow? Also, what size pump do you plan to use? The reason why is because certain pumps have different sized output and input requirements and if the hole in the tank is too small, it may put a lot of strain on the pump and affect the flow as well as the pump. Also, some people tend to use 2 points of suction depending on what size the pump is, to help minimize the amount of suction force they get from each point (bulkhead). Just imagine a fish swimming pass a single bulkhead that is sucking in water at 2000 gph! 
