Creating a biological filter
I am slowly trying to do my pond "right". We dug a pond 4 years ago. It holds around 2000 gallons. I know I don't have enough biological filtration. I have a pump that I think we calculated right for size, but it is basically being diverted around the pond and back in, without much in the way of filtering. The filter box catches the big stuff, obviously, but the nasty stuff/gunk is diverted into a "waterfall box" that I created myself, out of a piece of pond liner and bricks. It's basically a big depository on the side of the pond, which I built a waterfall around.
However, since it's not a self contained area, the water pushes through some bio balls and right back into the pond. I can fill that area with filter material like sponges/bioballs, but it has no "top". Any suggestions for what I can add to the top of this area to catch more gunk and nasty stuff? The bioballs do work to a degree, but any time I turn off the pump, this area empties, and so when I turn it back on, a lot of the gunk gets swept back into the pond by the force of the water. I need a better entry point back into the pond.
I'm including a link to a picture to help with this. I also suspect this area should maybe be bigger, or I could add a second pump and waterfall area to the other side?