since most of the population of bacteria lives in your filter, what happenes when u change ur filter media? do u need to do sumthin with the old media when replacing it?
most filters have multiple components like a sponge and a carbon filter or other things that can build up bacteria colonies on. So if this is your case you should only replace one component at a time....wait a few weeks and then change the other one. A lot of times you can get away with just rinsing the filter media instead of changing them with new ones. Make sure you rinse the media in the tank water...tap water will kill the colonies. Do this every couple weeks and you dont really need to worry about putting brand new ones in for like 2 or 3 months depending on how nasty they get.
In any other filter it should be ok because there still is bacteria in the tank, but for safety I like to do it in haves. I cut the media in half, and wait a week before switching the next one.