I agree with this, however I think that most aquarium setups lack proper surface area in their filter system to adequately house enough BB to do the job. So when the substrate is removed, it takes a large portion of the BB with it. Just a guess, though.
I bet your right. Just a bag of carbon? WOW! When you change the carbon,you are removing most of the BB. I have never bought a tank kit. That's good to know.I was talking about the basic small filters that come with the economy tank kits, like the top fin kit. They do the job but are often underpowered and dont exactly have much media to work with. I bought a 10g kit last year and the filter media literally consisted of a bag of carbon and nothing else.