if youre going to build it so that it sits under the tank, you will need a container (either a glass fish tank, a home made acrylic tank, or just a plastic rubbermaid container).
you will also need some kind of overflow (either drill the display tank, buy a store bough box overflow, or just build your own out of PVC)
and a return pump.
if youre going with a HOB fuge, you can build one out of an AquaClear HOB, by replacing the 110 impeller with a smaller (30 or 50) impller, and replacing the media with your live rock.
keep it simple... use gravity when possible. 10g(this may be larged depending on the size of your display, no fuge is too big) with single acrylic panel and a hole drilled and bulkhead applied. run a maxi jet in your sump to feed the fuge the water and allow the previously drilled hole to return back to your sump via gravity just my .02. make sure you allow a safe depth of water you dont want it too deep or too shallow. The divider is very important do not go without it, if you do you chance overflowing your sump if your water levels are not set properly. this divider will prevent too much from overflowing just incase the previous happens, always be prepared for spillage.
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