External filtration yes,but not open to air,under tank secondary aquariums .I'm talkin closed loop mechanical,biological,plus skimming,(not HOB,something a lot larger and DIY)Picture a 6 foot phosban reactor with differant media and backwashing capability,with a skimmer out the top.
You couldn't have a true closed loop with a skimmer on top.
Yes it is possible to do a tank with just live rock and a circulation pump. It just depends on how you plan to stock it for what it can handle.
Originally Posted by 11purewater
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