Some methods of aquariology, particularly those devoted to the culture of aquatic plants, make use of a space under the substrate called the plenum. The plenum in an aquarium may contain heating cables, pressurized water, additional micronutrients, and dissolved carbon dioxide from a reactor's output.
The normal goal of the plenum is to provide substrate circulation so that the rootlets of plants have access to additional water-born nutrients. The well-known system of under-gravel filtration draws water down through the substrate concentrating the waste of the aquatic organisms in the gravel for later human clean up. This tends to concentrate toxins at root level in addition to some nutrients. Better results for plants are attained by pushing the water up through the substrate very gently.