Live rock rubble is not effective in reducing nitrates, especially if they are small pieces. Nitrates can only be reduced with anaerobic bacteria, or with macro algae. Anaerobic bacteria will not grow on bio balls or live rock rubble, the rubble would have to be very thick in order for the bacteria to grow very deep inside where there are very low oxygen levels.

Lets talk about bio-balls, their function and why it is that you always find them where you find them. Bio-balls are often used in wet dry sumps, more often than not the drain trickles through them but they are not dirrectly submerged in the water. Why? The reason for that is because they need to be located where there are very high oxygen levels, where aerobic (not anaerobic) bacteria can grow. Bio balls are often used because they have plenty of surface area for this bacteria to grow. Alot of detritus and gunk gets trapped in there and with as much water running through them it gets dissolved back into the system. Bioballs need to be maintained regularly, rinsed in freshly made saltwater so that you dont kill the bacteria.