A mechanical filter is any kind of mesh material, sand, polyster stuffing,
etc through which water is passed and suspended particles of fish waste,
uneatten food, dust, etc

The term bio filter is a little bit of a decieving, it implies that there's
a special part of the pond where the bacteria grow. There's some floating
around in the water, some growing on the pond liner, the rocks. Mechanical
filters often can pull double duty, and be an a bio filter as well, just
because the filter material provides a high volume of surface area for the
bacteria to grow on.

A good bio filter will typically be a container filled with a light,
material that provides lots of surface area, compared to the volume, such as
floor polishing pads, landscaping fabric, screen material, bio-balls. Inside
this container you'd want a constant flow of aerated water.

Stagnant water or unaerated water creates an enviorment where anaerobic
bacteria grow, which creates the swamp smell, and doesn't work as well.
Aerobic bacteria on the other hand don't smell and work more efficiently.

Check out this website and get some ideas.
http://users.owt.com/jjspond/filters/myfilter.html


-S