I am setting up a 29 gallon tank and want to make a natural looking cave. I have been reading some articles on this and most of them say clay bots covered with rocks work best. Before I do this however I would like to know if I need to do anything to the pot before putting it in my tank as well as what kind of rock works best for this.
As far as I know you don't have to do anything to the clay pots just make sure to get the unglazed terra cotta pots and you will be fine. As far as the rocks you can use plan old aquarium gravel to stick to the pots using aquarium silicone.
You can make some very natural looking caves and ledges out of slate floor tiles. I bought mine at Home Depot. You basically just break them up randomly and then build your cave or ledge from the pieces and aquarium safe silicone. Just make sure your edges are not sharp. You can smooth them by tapping around the edges with a hammer or using a coarse file.