What would you make a good cave out of?

[a_c_arnold]

cement is safe, it just takes a while for it to cure. Soaking with frequent water changes will help leach out the compounds in cement that affect pH.

Here's something that I made out of field stones and cement to put in my aquarium. A cave could be made the same way.

 

[Dmaan.]

pot or 3 pieces of rock should do good!
2 for walls and 1 for a RUF lol.