The deer netting IS the filter medium for the bio filter
Quote:
Use 4 to 5 bricks or a plastic pop crate in the bottom of the barrel to create the clear water space. Fill the barrel with any available plastic material. 2 or 3 20X20 ft fruit tree bird nets @ $6.00 ea. Or a roll of 5 ft. X 100 ft. deer fence @ $14.00 will do nicely. Just wad the netting unto the barrel and weight it down with a flat rock. Aquatic plants in the top will help hide the barrel and will help remove nutrients from the water.

Source: http://www.doughtywatergarden.com/Tips/55%20Gallon%20Biofilter.htm

I figure by putting them in the pond they will develop microbes on them even before they are stuffed into the biofilter.