I believe you are wanting another filter.
One filter that is the proper gph is fine and dandy. But using just one filter causes a lot of poop to settle into one spot in the tank. Adding another, smaller filter(like a little submersible) can cause enough of a disturbance in the steady currents from one filter to stir up the stagnant spot and allow either filter to remove it.
All you gotta do is place the small filter in the tank so that it blows water towards the stagnant spot.
Hope this helps!