Or instead of window screening, you can use green house shading.
Probably the same stuff but, in the UK at least, seems easier to obtain.
You can also get a variety of mesh sizes.

Lee Brouillet wrote:
Kathy's suggestion of window screen is probably the best for both mechanical
and bio.
Lee
"John" wrote in message
m...

I just build a my filter box 40''x42x 60''(with plywood,treated
lumber,simpson ties etc) for my 6000 Gallon pond, need a lot of medium
to fill the filter, my budget is very tight now, please advice on any
good CHEAP medium ( I believe I need both machanic & bio filter
medium). thanks in advance

--
Matt Helliwell
www.helliwell.me.uk
matt at helliwell dot me dot uk