Michael- The idea behind biomedia is to provide the maximum surface area and the greatest possible mix of air and water. This promotes gas exchange and increases oxygen level. The lava rock pores will quickly fill with water and be no more efficient than regular rocks. Here is a link to some expensive bio-balls. You can use any kind of non leaching plastic that has a large surface to air ratio and does not bunch together. This link is for 40 gallons of bio balls just to give you a idea about the most efficient media.

http://www.marinedepot.com/md_viewItem.asp?idproduct=YRB1377

Please no more rain for a month! :|