The best filter I ever "built" was one of those watercourse things from HD. It has three (I guess you'd call them) chambers, or areas of flow. The first had lava rock and gravel in an old t-shirt sown into a bag. The second had more gravel, and the third had waterhyacinth. Pretty much took care of bio and mechanical filtration. I was using a 1000gph (at 0 feet) pump with a sponge pre-filter, and 1-1.25" tubing. All this in a 125gal pond. Water quality was perfect.