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Originally Posted by Delphinus
I always thought it was just a case of the bioballs were an aerobic zone and you need an anaerobic zone for nitrate reduction.
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Kinda. Nitrate reduction can occur in aerobic zones. For simplicity sake, the bioballs house Nitrobacter, which produce nitrate as the end product of thier biological process.
However, some of the organisms in live rock further processes some of the Nitrate into Nitrogen gas. The gas difuses into the water, and escapes into the atmosphere, or sometimes collects within the sandbed (even shallow ones, in the upper aerobic zones) and escapes to the atmosphere when it is disturbed and rises. End result being lower (but not no) nitrate after nutrient minerilization.
Bioballs directly compete with live rock for the available nutrients thereby limiting the nutrients available to the organisms on the rock, obivously impeding thier growth.
Bioballs are highly effective at removing ammonia and nitrite, and in certain applications are necessary or provide a level of insurance. IE lots of bioload, not a lot of rock.