Forgive me if this topic has been covered -- I did a search and did not find anything specific to lava rock....though a lot of comments on the topic. This thread is to discuss advantages / disadvantages of using lava rock as the media for biological filtration. I have a new canister filter that did not come with adequate bio-media. I will take as a given that the all-too-expensive specialty bio-media is probably superior to lava rock in applied mathmatics.... ...but in practicality....is there any reason not to use cheap lava rock in a canister (or similar) filter system rather than running out and buying bio-balls or similar? And of course the flip side...is there really any reason to buy expensive bio media except where space is highly limited (thus the absolute best efficiency required)?