A freshwater "plenum"
A freshwater "plenum?" In an undisturbed planted aquarium there are some advantages to keeping nutrients confined to the substrate and out of the water column. Low levels of phosphate in the water don't encourage algae, for example, and low nitrate levels work for the long-term benefit of the fish. Chemotrophic nitrifying bacteria, the ones in the bio-wheel and the gravel, are competing with plants for available ammonia. Though too much can be made of this competition, once the nitrogen of ammonia has been converted to nitrite, it is all but lost to plants. Bacterially-generated nitrite is converted to nitrate, an endproduct of aerobic bacterial activities, and in spite of the activities of a range of bacteria involved in de-nitrification, it ordinarily builds up in the aquarium. I've learned from reading Diana Walstad that apparently plants don't begin to tap into nitrate supplies until the local ammonia supply has given out, for ammonia is metabolically more easily available. Exporting nitrate accumulations becomes the fishkeeper's responsibility, one major motivation behind water changes.
A decade ago there was a flurry of interest in adapting to freshwater planted aquaria the "Jaubert" system of reefkeepers, which establishes an undisturbed anoxic zone isolated beneath gravel. In this plenum space de-nitrifying bacteria can metabolize nitrates, hopefully rendering them back to gaseous nitrogen, which remains dissolved in water and is eventually outgassed into the atmosphere at the water surface. In the Jaubert system, it is not the empty volume of anoxic water that counts after all, for water supports few bacteria, compared to the surfaces that contact it; it is the adjacent gravel surfaces that provide strata for the anaerobic de-nitrifying bacteria involved.
Cercona, a company that makes sintered glass products of recycled glass, was introducing rectangular slabs of the chemically inert ceramic-glass-foam material it called "Cell-Pore," with interconnected pores of 50-400 microns. Because the material is rigid, you wouldn't be likely to inadvertently disturb the anoxic water stabilized within the pore system. Nor could sand-moving fish able to destabilize the isolated anoxic plenum. This new material was enthusiastically reviewed in F.A.M.A., February 1999; then the rights were bought by Kent Marine, then sold, and the product evaporated.
The concept still interests me, though I suspect de-nitrification is already happening in the anoxic zones of my undisturbed substrate and in the lower zones of mature biofilm.