Which materials are suited for building caves?
which materials (except stone and wood) allow building hideaways with very small openings?
generally, anything that allows drilling holes or being modeled and does not release toxic substances is suitable. Non-enameled natural clay (without additives) is particularly well suited, this allows making caves in all imaginable shapes. The clay, however, must be baked before putting it into an aquarium!
Accordingly sawed coconut shells are also suited, and many fish readily accept them; the edible flesh of the fruit must of course be removed entirely.
Dr. Bodo Schnell