A very good finish filter is Kent Wallace's up flow sand/gravel filter. This has to be pump fed so it would sit at your water fall. It will add bio capacity and polish your water. Here's a construction thread from another forum.
http://www.*******.com/forums/showthread.php?t=36550
A good bio filter that is as cheap as you can get is gravity fed 55 gal barrels with poly package strapping in them. A 9,000 foot roll sells for $28 and will fill two barrels. Use uni seals where you can, they are cheaper than buldheads.