The number of nitrifiers depend on the load of the tank. They will spread on all surfaces they don't really care for the location as long as they get what they need. You might have alot of surface area and/or other ways of getting rid of nitrogen like plants, so you don't notice it if you killed the ones in the filter.I do strongly feel that even if cleaning my Bio under tap kills off all the BB from the filter, there is sufficient amount of BB within the tank (on the glass, substrate, rocks, driftwood, and decor). We also preach overfiltration which makes me think how much BB is already within my system? Am I able to get away with breaking one of the fish keeping commandments because there’s plenty of BB within my tank?