Lava rock is another bio media that bacteria can colonise on. By looking casually at a lava rock, it actually looks like a sponge. if it has the structure of a sponge, it would have been a much better bio media. However, it is not. It just looks like it and the similarity stops there.
Lava rocks have many holes (air pockets) throughout the rock but they are not connected and does not allow water to flow through them. As a result, water does not flow through lava rock. Water that flows pass lava rocks just travel around them. Due to their rough surfaces, they have slightly more surface area than a normal piece of rock of the same size. However, they are have nowhere near the surface area efficiency of ceremic rings and in fact, even less than bioballs!
Another disadvantage of lavarocks is that they trap dirt and water suspended particles especially easily. When they get clogged up, they are difficult to wash or handle due to their sharp and abrasive surface.
Because their surface area is all on the outside and water does not flow through the rock, a large piece of lavarock will be taking a lot of space. And extremely surface area inefficient.
If you are stuck with lava rocks and you absolutely have to use them, break them up into as small pieces as you can to increase the surface area-volume efficiency.