Agree, mature media (MM) is pretty effective virtually anywhere inside your filter. The autotrophic bacteria will not have far to move to continue establishing new colony outposts. If you can, you might want to try to optimize it. Most new filters come with more than one type of media (mechanical, biological and chemical.) A typical example would be sponge (for coarse mechanical and biological filtration,) a carbon section (for chemical filtration,) and a floss pad (for fine mechanical filtration.) Typically, the water flow would move through the course mechanical first (the sponge,) then on through the carbon and then finally the floss pad.
The MM would be most effective at the very beginning of all this. If you position it in the water flow prior to the sponge then the water movement itself will aid the bacteria in moving from the old media immediately into the new sponge, which will have the most optimal surfaces for them to attach to.
Incidently, in my example we would often recommend that the carbon is not needed on a regular basis. Carbon is best at doing special temporary jobs in the filter, like removing medications, tannins or a few other things. It gets used up in about 3 days typically and is expensive to keep replacing. Its a great media to have "on the shelf" though.
My simplistic example also ignores how creative you sometimes must be to cram in the MM into the place where you want it! Sometimes you have to use scissor or otherwise break up the old media so that it will fit reasonably well.
~~waterdrop~~
