-put a pump in the bottom and build a cage around it with egg crate
-layer scrubbies and filterfloss above the pump
-dump in a bunch of scrbbies/bioballs/magic rocks
-put a layer of japmat on top of everything
-drill some holes in a piece of pvc and use it as a spraybar
-add some sort of a lid to contain the splashing
-done
if you really want to go nuts you can drill a ton of holes in another piece of pvc and stick a heater in it.. just be sure to put it down in the bottom so it always stays submerged