I have been running bio balls for three years in a 100 g reef. I am running a 4 inches of live sand.
Bingo. Filter A (bioballs) produce nitrate because of the highly oxygenated enviorment and Filter B (the DSB
) works on removing those nitrates thanks to the low oxygen regions deep within the sand bed. Out of curosity what is your nitrate reading?
The reason filter media, sponges and bioballs produce nitrate is that they are oxygen rich and thus fairly effective at ammonia and nitrite removal but the nitrate can not be broken down since the bactera that utilizes nitrate prefers lower oxygen levels.