2-inches of dry sand is a sufficient cap for a dirted aquarium. This is thick enough to prevent too much ammonia from leeching into the water column. It will also create an anoxic zone on the bottom layer to promote the growth of anaerobic denitrifying bacteria.
Denitrification
Increase denitrifying bacteria in an aquarium by creating a deep substrate bed of at least 3-inches. The bottom inch will contain no oxygen and will allow denitrifying anerobes to cultivate and utilize nitrate instead. Sand or gravel substrate can be used.
Nitrate can be naturally removed from aquariums with denitrifying bacteria. A 3-inch deep substrate will create an oxygen-free zone that allows anaerobic bacteria to convert nitrate into nitrous oxide and nitrogen gas. These gases then escape the tank into the atmosphere.
Aerobic nitrifying bacteria convert ammonia (NH3) into nitrate (NO3). Anaerobic denitrifying bacteria convert nitrate into nitrogen gas (N2). The key difference is that aerobic bacteria require oxygen while anaerobic bacteria must live in environments with little to no oxygen.
Anaerobic bacteria removes nitrate from an aquarium by converting it into nitrogen gas, reducing the need for water changes. It also produces toxic hydrogen sulfide gas (H2S), which concerns many fishkeepers. However, H2S is safely converted into harmless sulfate in the presence of oxygen.