The tank is overstocked. Adding fish = more nitrates, adding food = more nitrates. The upside being that you know your tank is properly cycled (unless ammo or nitrIte are also rising)
As for the filter media, never change the bio-filter (in a kit, usually plastic brillo pad-like). As for the charcoal pad, it really depends on how quickly it gets dirty. If it's not so gunked up that the water is flowing around it rather than through it, or otherwise impeding the flow of the filter, there is no need to change it. For a beginner, it may be best to do it once every month, but after a while you will be able to tell when it actually needs changing, and as a result, usually save some money on cartridges.
My advice, is that if you are thinking about buying a second filter anyway, just replace this filter with one that has a higher capacity, and that uses sponges that can simply be rinsed, as opposed to cartridges that have to be purchased over and over. Rinse your sponges once a month (or whenever you can tell they need it) and you should be golden.