, I completely agree with jsoong and JProx about planted fuges as filters, provided there are plants in the main tank. Respectfully, I think there are a couple of good reasons
to run a planted fuge, even with high light main tanks. Since we are dosing nutrients to total water volume anyway, why not throw, at worst, a desklamp or two down there and grow out stuff or provide plant shelter for a breeding colony of shrimp or something? It is certainly unnecessary, but still interesting and only limited by imagination. Cheap to mess around with, too.
I have no experience with brackish, but think it could be cool to breed something in there for food. Alternatively, you could put brackish-compatible plants down there to adapt, if for some reason (say, the cost of lighting for the main) you do not want to do it in the main tank. If the main has rowdy animals, you could attach java ferns to rocks and stuff down there before transfering them over. Just thoughts.