So what do you use for media? I've got a Penguin 125, that has the disposable cartridge with carbon. Since I'm cycling the tank, I've put an old Top Fin cartridge (that had been kept in my 10 gal) in the space behind the Penguin cartridge. I suppose I could just toss the Penguin cartridge and fill that entire space with foam or floss for mechanical and additional bio filtration.