A sump can be described as a few things. Place to put your skimmer, heater, carbon ect. Also a place to do your water tests without disturbing the main tank and it also addes to the tank volume.
The overflow is to feed the sump water and skimm the surface.
Closed loop is to add more flow to the system with just a pump and the plumbing. You don't need a sump for a closed loop. Hope that helps.
Clinton
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