Those are really nice caves for them. I've looked for the lava rocks from local LFS but seems like most of them only have like small lava rocks. I read and understand its best to avoid leaning the lava rocks on the glass wall to avoid putting too much pressure and causing leakage from @odin post, therefore smaller lava rocks might not work i suppose? I'll probably take a look at those "ceramic caves", might have better luck on them.