I used lava rock because of how much surface area it has and since it is very porous I thought it would make a great bacteria growth area. So far I don't beleive I have had any problems with it and it is definatly far cheaper than bio balls and live rock. I have it in a 5 gal
water cooler jog flipped upside down with water flowing over the rock through the jug. About 1/4 of the jug is fully submerged while the rest stays out of the water with just the tricle from the overflow.
About the only known issue I have with the rock is that if it isn't rinsed well before you put it in, you get a very fine red "dust". This does settle and the water stays very clear after that, but I do have a good bit of it in the sump and doing water changes it gets stirred up and has put some up in the display.
Other than that I can't say it has been too bad.