Two-layered substrate

A two-layered substrate. Think of your substrate as two layers, with different functions: an upper layer and a lower layer. The upper layer of substrate is usually formed of coarse sand and gravel, without nutrients that might leach into the water. Larger pebbles tend to rise to the surface in time, like the stones in a New England field; silty stuff tends to get buried. Only continual stirring could retain an unnaturally homogenized mix, but I'd caution you that gravel vacuuming should be minimal; if gravel vacuuming is unavoidable, try not to go so deep that you cloud the water. Besides avoiding nutrients in the upper layer, you also want to avoid Ca and Mg carbonates, like aragonite or limestone and stick with chemically stable, fairly uncomplicated elements like silica.
 
The lower layers of the substrate are going to vary according to what you intend for the aquarium you're establishing. The remaining pages in this section cover some substrate materials.