It populates any porous surfaces where food is present. This includes Rocks, Sponges, Filter Media, etc. The BioMedia is manufactured to give them a lot of surface area to grow/populate. That doesn't mean that they aren't everywhere though. It is true that they will only populate to what they can sustain, which is why I said remove bricks first, wait two weeks to let them grow in number again taking up more of the surfaces that already exist, then remove the gallon of balls, and they will repopulate to the rock/sponges, etc.I thought that the majority of the "good" bacteria resides in the media bricks?