I think its that the fish are well fed & pretty much continuously passing NH3/4 from the gills, so there's always that to feed on, & the NH3/4 is removed from the system rather than being nitrified into NO3My comment on the nitrate was something that was explained to me by an Ogo farmer. When I took his advice regarding nitrate my cultures of ogo and chaeto stopped crashing. Perhaps your scrubber is more efficient at nitrogen fixation than our macro cultures in a vat style things we're doing...or maybe when you were getting a hobby test kit reading of zero you had more nitrate than when I was getting a hobby test kit reading of zero...or maybe I'm completely wrong and nitrate had nothing to do with my chaeto and ogo crashes.