Biological filtration is the best IMO. My fiance has a 46 gallon bowfront. It is a FOWLR. The fish include: 2 true percs, hawkfish, flame cardinal, bi color angel, and spiny box puffer. There are also 5 different starfish and 24 urchins. The tank is fed daily and the puffee is fed daily. The filtration on the tank is live rock in the display and a 10 gallon sump with live rock only. No skimmer, no carbon, nothing but live rock and live sand. Now the puffer has been in the tank for a little over a month now. We have been working on getting the internal parasites out of him by trying to feed him as much as possible. Now he tends to not each all the krill we give him. Alot ends up in the sump. Well I have not done a water change on her tank in over two months because I have been working a lot. So I was curious last night and I tested the nitrates of the tank. I was waiting to see the nitrates in the 100s because of all the food and no water change. Well her nitrates where at about 50-60. Which yes that is high but for a fish only it will be ok. But yes I need to do a big water change. I was trying to figure out why the nitrates would be so low with such neglect toward the tank. Now this tank has been up and running for a year and half now. So I look in the sump and all the live rock is just completely covered in sponges. White sponges and black sponges. The black sponge on one rock has grow so much it is now growing on the glass in the sump. I am just completely amazed on how nature will take care of it self and had to share. Will post pics of the sump in a little.