Sandstone and slate are very stackable and readily available at any quarry or stone supply. This is my 210g with 30 Africans...Malawi, Tang. and Mbuna etc. They are all hiding in the picture from the Oscar that cruises the upper level of the tank. Caves and hiding places are essential for success in raising Africans. Also, I have had more success with a coarse sand vs. gravel as these fish LOVE to dig...so dont stack rockwork on the sand, move it out of the way and set the rock work on the glass(carefully) or you could end up with an avalanche!! Seriously.