A second filter will hardly help you if this is that overstocked tank. Bacteria will only multiply when you add an extra ammonia source. If your tank is in fact overstocked, you do NOT want to add another source of ammonia. Nitrifying bacteria will not remove nitrates, and I am sure you already have a bacteria colony converting waste into nitrates. Water changes will remove some nitrates, but that will not change the fact that you are short on space for more fish. Another filter will pretty much only help with mechanical filtration, which won't really benefit much in itself.