It is an interesting part of my system. It started as a way to add water volume so I could put more in the display, feed more etc. but it has kind of duplicated the deep water environment in the ocean. They are plumbed in series. the first one, the sides are now completely covered in white sponges. The second one is now covered with tube worms. The interesting part to me is even though the environment is the same there are no tube worms in the first and no sponge in the second.
I run outside air through bubblers to help with the PH so it is not an oxygen issue. I am not sure what to make of it but I figure it must be adding to the filtration. The only problem is I have to syphon out the bottom every other month or it starts to form a dead zone and then the ole rotten egg smell is there when I do syphon. I was tempted to leave it because I figure it must be consumming Nitrates but the thought of poisoning the system scared me.