Best bio media can be made from a variety of materials, including lava rock. Lava rock is porous and has lots of small nooks and crannies that help it trap impurities as the water passes through the aquarium filter. Lava rock should always be rinsed very well before use to remove dust or debris that could clog the pores and interfere with filtration. You can also soak it in a diluted bleach solution to help kill any harmful bacteria.