How Quick Does Bacteria Multiply In A Filter?

[jrussuk]

I'm changing my external filter this weekend. I'm going to put as much old media into the new filter but was wondering how quickly does bacteria multiply in the new filter?

 

[KittyKat]

They approximately double once every 1.5 days. Most external filters for the same volume tank have about the same media space, so you should not have anything to worry about. The potential scenarios are either that your should be able to fill the whole filter with old media and have some new media on top (no effect on the tank), fill the filter perfectly with old media and no new media (no effect on the tank) or not be able to fit all old media into the new filter (and hope that the media already has enough bacteria for the existing stock).