I have an eco-system on each of my three tanks. Like I have said a hundred times, this is what works for me, and it may not be for everyone. The only downfall for people seems to be the price. Many suggest building their own, but for me, it was easier to just buy them. How it works is this. The water comes from the tank into the first chamber, which is filled with bio-balls. This first chamber filters out the large debri and stops and bubbles from going any further. It empties (at the bottom, so the water is forced through all of the bio-balls) into the second chamber, which has the miracle mud (MM) and macro algae in it. This is also where I hear alot of negative comments, about the MM. Eco-systems claim is that the MM, along with the macro algae, break down all of the nitrates. The algae also produces oxygen and decreases the phosphates. It is because of this chamber that there is no need for a protein skimmer. The water then goes into the third chamber (over-flow at the top so that again, the water is forced from the bottom to the top, eliminating any still areas), which is also filled with bio-balls. These bio-balls remove any remaining bubbles and prevent any of the macro algae from entering the fourth chamber (from the bottom, to force the water through all of the bio-balls), which is like a clean chamber. It is in this chamber that the return sump is attached. You can build your own, but they have designed theirs wonderfully, with water movement and bio-medium contact time in mind. A lot of people also complain about bio-balls being "nitrate factories". All I know is this. I have 3 tanks, 75, 90, 120. All chuck full of LR and inverts. I use no skimmers. I keep my kH and Ca levels optimum so the live rock can function at a maximum level for filtration. My nitrates are 0, phos 0. I have never heard someone that has used it complain, just people that don't and think that they can build their own. For me, I wouldn't use anything else because it works perfect.