I prefer mostly natural decoration for my volcano shrimp (opae ula) tank.
My favourite piece of decoration is a piece of natural lava rock.
It has a beautiful texture and is light weight and comes in a few colours like black, brown or maroon and lightweight.
The holes in the lava rocks provide a good hiding place fot the volcano shrimp (opae ula)
Pregnant volcano shrimp are often found hiding in there.
If you unable to purchase big pieces of lava rock, you can also use small lava chips too.
I love to use them for my miniature and nano size volcano shrimp (opae ula) tank !