custom-sized filter medium

I'm building a custom-made fish habitat structure, kind of like
NetMax's styrofoam scultpure (which is way-cool in my book), only
different. I intend to utiize certain integrated structural elements
of the structure to house a biological filter medium. The housings I
have in mind will consist of two (2) 4"-diameter cylinders, approx.
18" long. I'll use a power head to circualte water through the
housings.

The questions:

1. What are biological filter media typically composed of? Synthetic
sponge? Are such media avalable in bulk sizes that I could utilize?

2. What would be the best medium to use to fill the housings as
described?

3. Is it feasible to pack a cylinder of that size with with filter
medium?

Thanks as always, and respectfully,
-Zz