They can be built that has a easy, solid base to produce them strong, but some prospective buyers have claimed the ornaments are a little bit light and might occasionally topple about. This might rely upon the depth of the tank – much larger tanks might not have this problem. https://www.mediafire.com/file/hdr3ghr12b1dvq1/Essential_Services_and_Products_for_a_Healthy_Aquarium.pdf/file