The organisms do not care about the

The organisms do not care about the sediment mineral composition, only the particlesizes and shapes. Most aquarists use the commonly available aragonitic sands to "providea calcium reservoir." Additionally their bright white color is often aesthetically pleasing.However, if the system's pH and calcium concentrations get low enough to dissolvesignificant amounts of the sediment there are some very severe problems and all thesand in the world won't help. Very successful tanks may be set up utilizing black lavasand, or fine siliceous sand, as long as the grains are of the appropriate size. There issome concern that siliceous sand will fuel diatom blooms, but such blooms may becontrolled by the appropriate grazers. There is absolutely no need for any subsurfacesand structure such as a "plenum" or shelf. In fact such structures will reduce thesediment volume that is available for the bacteria.