[Dentoid]

The top layer of sand should be stirred or sifted to allow for oxygen and an aerobic layer. This can be accomplished using sand sifting organisms, like star fishes, some gobys, sea cuckumbers and others. Or you can physically stir the surface with a small aquarium rake or when vacuuming the substrate. The bottom layer of sand is the anaerobic (without oxygen) layer that is nasty but harbors the bacteria that breakdown nitrate to harmless nitrogen gas that slowly bubbles out of the system. Thus nitrate reduction. The same thing can be achieved using a 5 gallon bucket filled with sand to within 6-8 inches from the top and have water exiting your tank slowly run over this sand and back into your tank. However, the refugium is the best of both worlds. You can have 4-6 inches of sand and nitrate reducing macro algaes in one place. As well as a place to grow pods, a natural food source, for feeding your tanks inhabitants. Another benefit of the refugium is that you have a place to put organisms that you find in your tank and don't want there.

It's not a bad thing, but like everything else it needs to be maintained.