Pond filter: how is sand/gravel?

Has anyone built a sand and gravel filter for their koi/goldfish pond?

The stats:

	- pond is 5' x 10' x 1' to 2' deep
	- about 20 koi/goldfish from 1" to 8",
	  about 100 guppies (increasing by the minute!)
	- fed with commercial koi food twice daily, some
	  nice yummy earthworms whenever I dig 'em up
	- variety of plantings: water lilies; submerged
	  (Elodea/Anacharis, Ceratophyllum, Cabomba);
	  emgergents: Iris, Typha, Lizard Tail, Thalia, etc.
	  Water hyacinth floating
	- all plants except floaters, submergeds in plastic 
	  pots with gravel layer on top to decrease turbidity 
	  from soil substrate
	- submerged centrifugal pump: 1,200 gallons/hour
	- homemade submerged filter, about 1 cubic foot
	- filter medium is 3 different kinds of plastic 
	  sponge-like foam sheets of varying densities
	- filamentous green algae is prevalent but not problematic

The Problem:

	The plants are thriving, the fish are gorgeous, the water
	is clear  

	it takes several hours and much scrubbing once or twice a 
	week to clean the crud out of the filtration media.  

	I realize that this could be alleviated with a larger filter
	but the messy and tedious task of rinsing the foam pads
	would remain.	

My goal:

	To devise an above-ground mechanical filter that's much easier
	to maintain.  

	One idea: construct a sand and gravel filter similar in concept 
	to those used in older swi