Hmm... I thought the main reason for for bio-balls and coral chips being called nitrate factories is because they do not have anaerobic region whereby de-nitrification bacteria can thrive? Without these anaerobic bacteria, the other bacteria can only convert ammonia to nitrite and then to nitrate and stop there. The denitrification process in the main tank by the LR and sandbed cannot match the speed of the amount of nitrates produced by the bioballs etc, hence the term nitrate factories. Correct me if I'm wrong.. this is what I have found out from readings.... and of course those detritus plays a part as well