If you can picture a tank, with nothing more than a few live rocks & sand, plenty of micro organisms, small crustacia, and worms, with plant life you would actually need to thin out every month or so, you have pictured a refugium, a secondary tank that fish and larger organisms can't infiltrate nor disturb and disrupt, and that is what your aiming for. This tank essentially cleans all the nutrients from the water, not only that but if you have the lights on when the lights are off in the primary tank, you can keep the PH stable when primary tank lights are out.
This will also prove valuable in feeding your fish NATURAL and live foods out of your refugium, be it plants, or small animals or even feeding the corals with the offspring of these organisms that are small enough for most coral to intake and will flow right out from the refugium into the main tank without needing to be introduced manually.(phytoplankton–microscopic plants and bacteria & zooplankton–microscopic animals) Add all this to having more water that will turn into a more stable water system, a refugium is more than just a filter, it is a filter, food source, and makes your water alot more stable because it is now a larger volume.
With this in mind, I do not see how you could possibly do this in a bio-ball type compartment.
Hope this gives you a better insight.
