People argue that bio balls become nitrate factories, but i have never understood the argument. IMO
, using live rock in its place would catch just as much debris that could lead to nitrates. I have had two acrylic aquariums with the built-in filtration in the back and never had a problem with nitrates. I do use live rock in the tank too though. I do think that live rock with a good skimmer is a good way to go. I'm just not as anti-bio balls as they have never done my tanks wrong.