Pick the biggest filter capacity you can afford. Making sure, you are not stuck with having to buy alternative media from the manufacturer. dumping the carbon will free even more space for media. a bigger pump will also allow a good side spray bar for oxygenation. set the filter up as it comes, or as recommended. put both filters to work. this will take some time. meanwhile do some research on alternative media for your filter, reading threads can be a good thing. ask a few questions, if people are using different things try to find out why. this will help you decide if changes are needed! if so do them bit by bit.