Food particulate is caught in the stage 1 tray and decays. Ammonia is produced but so is bacteria that live in the filter that live on ammonia and create nitrite. Other bacteria live there that eat the nitrite and create nitrate. The bacteria that eat nitrate and produce nitrogen live in anaerobic areas, deep recesses of LR
or the bottom layers of a DSB
. You have no oxygen poor areas in the cannister.
Same thing happens in the Tidepool. These are great for FO
tanks because they are great at the first two stages of the Nitrogen cycle. Nitrates are non-toxic to fish in moderate quantities but corals can only tolerate miniscule amounts.