Ya I have always had large lava rock caves in the tank but I have never used smaller pieces (which I have tons of lol) too small to use in the tank I have broken them up and filled a basket in my new filter aquatop cf300. I will use all the bio rings from my old fluval 204 and in the other basket and the bottom basket will have course and fine sponges in it.
I use lava rock in all my sumps instead of store bought garbage. It works incredibly well and has fantastic porous qualities. I have used it in all of the custom sumps and tanks i have built. Disclaimer i have a side business where i build custom tanks and filtration systems from 100 to 3000 gallons.