A concrete-based mixture like aragocrete or hypertufa would work well. You could also carve a cave out of insulation sheathing and cover it with grout. With any of these methods you must be sure the thing is cured and leached before putting it in your tank.
i made my rock caves insane by cutting off their eyelids and forcing to stay awake for weeks at a time and playing loud music.. constant beatings also helped to push them over the edge... a strobe light was also helpful...