dumb question..what is a closed loop?

[hipertec]

Im getting a 180 gallon with 4 bulkheads on top. I heard to go closed loop and dont know how or how it works. Can someone help?
Is this the best way to set up my system? it has a center overflow with 4 bulkheads evenly spread on the top of the tank.

 

[old salty]

A closed loop is a way of circulating water; instead of using huge powerheads and the likes. It's closed loop, meaning the water exits the tank, goes into the pump, and then back into the tank. Essentially, the water will not contact the air. Check out some of the large tanks on these forums. You can get an idea of how a well designed closed loop really takes care of tank circulation.