I'm interested in denitrification to get rid of or maintain a low level of nitrate. Would like to know how I can increase the population of these BB.
Many aquarists neglect denitrification, but as you seem to realize it is an important part of the biological system.
The bacteria responsible for denitrification are heterotrophic, meaning bacteria that cannot synthesize their own food so they need organic material such as fish waste, dead bacteria, fish and plant matter, etc. While some are aerobic (which means, requiring oxygen to be present), many are facultative anaerobes, meaning that they can survive in either the presence or absence of free oxygen. Anaerobes are organisms that do not require free oxygen for growth. This has significant consequences in aquaria.
There are several species of heterotrophic bacteria that utilize nitrate by consuming the oxygen within nitrate and releasing nitrogen gas. They do not require free oxygen in the water so they are facultative anaerobes, and generally occur in what we term “dead spots,” which occur when water movement is stopped and thus no oxygen is available. These are the good guys among heterotrophs, since de-nitrification is important in a healthy aquarium. And they will naturally occur in the lower level of the substrate, which is a good reason for not disturbing it with deep cleaning, at least in several areas such as under chunks of wood and rock.
The greatest population of bacteria in a healthy balanced aquarium occurs in the substrate, not the filter. The floc or humic compost that collects in the substrate is the host for the biofilms; this is why the substrate in planted tanks should never be disturbed, and many aquarists apply this to non-planted tanks as well.
In very general terms, aerobic nitrification takes place in the top 1-2 inches of the substrate; anaerobic de-nitrification takes place approximately 2-4 inches down, and anaerobic bacteria producing hydrogen sulfide occurs in substrates deeper than 3-4 inches. In all three cases, it will be deeper in coarse substrates (like pea gravel) and more shallow in finer substrates such as sand. These generalities will also vary with the presence of live plant roots and substrate “diggers” such as snails and worms, since these factors result in more oxygen being made available in the substrate, reducing anaerobic bacteria activity. An oxygen level in the substrate of as little as 1 ppm promotes nitrogen reduction rather than sulfur reduction (hydrogen sulfide).
Having explained the processes (I hope), the answer to your question is to leave at least some of the substrate alone. I have tanks in which I never vacuum the sand at all, and some tanks I do just the top of the front sand. I also have a great many chunks of wood in my tanks, and the sand under these is never disturbed.
Byron.