Natures Ocean AM0825 Coral Base Rocks for Aquarium 40lb bark spray collar The top is solid wood The top is solid wood There has never been a pet fountain that looks or works as well as this one does Nature's Ocean Coral Base Rocks 40LBS (Packaged: 2 per case 12 Inch 17 Inch)Nature's Ocean Base Rock is 100% premium all natural aragonite rock covered with naturally formed holes and crevices which makes for some very interesting and unique shapes and is easy to stack when creating a base for growing live coral or as an addition to existing coral based ecosystems This porous characteristic provides a natural biological filter as well as a perfect environment for denitrifying marine bacteria to grow and flourish