What can a refugium do for my tank? FYI

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Fuge. Refugium. A small green tank. Green in various senses.
It is either part of your sump (the central chamber), or is set up on a closed loop with your tank.
OK: definition: 'tank on a closed loop'---simply put, you put a maxijet pump in a second tank, clamp the input AND the output hoses firmly into place in your sump OR display...