From Craig Hahn, veteran pond keeper and all around handy guy:
Just thought I might mention a solution that I have come up with - once the ice is off the pond here. (More snow here yesterday. yuk!)
I have a half wine barrel (~20gallons) that I fill with 3" lava rock (to handle the inflow) then use that fiber material that they sell for pillows - still can see the look of the sales lady when I told her I was using this for a bacteria medium). I tip the barrel so the water spills over into the pond and cover the top of that fill material with a layer of foam thick enough to lay flat. I cover this with small - medium rocks to make it look nicer. Then the pond water is forced into the bottom (lava rocks) and is filtered and nutrients consumed when it warms up a bit by the bacteria (I don't add any product - just use nature). You could clean the filter material if you like after the water is cleared but I don't - I love the natural way of things.