This past weekend I installed a small (20" round?, used, and FREE) pool filter on my 2500 gallon pond. I used 100 pounds of sand filter media, as called for. I don't know if this is a good thing or bad...it's cleaning the water, but needs backwashing constantly. Can I expect this to diminish once the water cleans up some, or should I replace the sand with something else (rocks, pebbles, LEGOs)?