Kathy's suggestion of window screen is probably the best for both mechanical
and bio. If you just wanted bio, then the strapping material used for boxes
has a lot of surface and is easy to clean, as well as being relatively
cheap. You could put a goodly chunk of window screening in the first half
(water inlet) and some of the strapping material on the other end (outlet)
and get a considerable amount of surface area for your biobugs.

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