I really dislike it when people make statements that almost all the BB is in the filter and will even state percentages because as far as I know there havent been any scientific studies done.
Welllllll.....there is an issue with this. I mean..not really an issue...not sure really how you would classify it.
While BB lives anywhere and everywhere in a fish tank, they are more abundent in your filter media than anywhere else in the tank. A couple of factors here. First is water movement. BB thrive best when there is constant water flow (I.e filter) water is contantly being moved and bringing goodies to them. Now...inside the tank itself, it is suseptable to "dead spots". By this there is little to no water movement. While yes there will still be BB in these spots, it just won't be in the same numbers.
The second factor that comes in to play is surface area. Now I know what your thinking. How can one say that a filter offers more surface area than that of what is inside of the tank.
Let's break it down. We'll start with the substrate. Let's say your average bed is 1.5 - 2" deep. Of this...how much is actually "exposed" to water movement? Only the top "x" amount. The rest is stagnant. Yes, BB will be present but not as abundant as top layer. The deeper you go, the less in quantity there will be. Most of the good "goodies" will have already been devoured by the top layer BB leaving only morsels for the rest to fight over.
Now we compare this to that of filter media. What is filter media? floss/fiber. thousands and thousands of strands. Each of these strands are exposed to the circulation of the water. The filter is constantly pulling the "goodies" from the tank and passing it on to the BB. With the water circulation, all of the BB get their fair share of "goodies" and thus thrive and survive and reproduce more rapidly.
On to the percentages. Now if someone can sit there and tell me that "x%" of BB is here and "x%" is there...I say one thing...prove it. I'm not a scientist or biologist but I would like to see facts. It's all a matter of concentration.
Let's pose a couple more scenarios, we take a 10g tank and for practical purposes, we'll say we have 12 neons in this tank. Standard 10g rated HOB. Tank is cycled, well established and has been running for quite some time with just standard water changes. No further maintenance. Time comes to change the media in the filter. What happens? Chances are you will find yourself in a mini cycle. Whether you see the tell-tell signs of it (i.e. cloudiness), it will happen. Why? Because you have just removed a large chunk of BB from environment. Now there is not enough BB to keep up with the bioload.
Same scenario, except this time, we are tired of the gravel and want to switch to sand. We remove all the gravel and replace it with sand. Once again, we have removed a large portion of the BB. Large enough to cause what is in the filter media to be ineffective. What happens? Same as above...a mini cycle. The BB have to grow enough to support the bioload.