I am trying to get my 20 gallon ready to grow live plants. I have started making my own filter cartridges to do away with the carbon that comes in the store bought cartridges. I currently am running a regular bought in the store cartridge with carbon in it and a white 'bio-fiber' pad. The water (in order) runs through the blue filter cartridge then through the white bio pad (not sure if that makes a difference). My question is, In order to not crash the natural biological matter in the filter which should i replace first? I will be running the old media (whichever is more beneficial) with the new home made filter pad for a week or two, in order to give the new filter pad a chance to catch up.