Biowheels are great, but when the bioload is large (as when overfeeding or when stocked with lots of fish), then the ammonia/nitrite is converted to nitrate extremely efficiently by them. This on one hand is great since you have great aerobic biofiltration, you now have lots of nitrate in the water column, which can only get converted to harmless gases in the deep parts of LR or in the DSB. To get nitrate from the water column into the deep LR or DSB is more difficult than if you are trying to get nitrate produced along the LR surface or in the LS.
kris