What you are saying is correct. However the debate around DSB's is the process by which the water is moved within the sandbed and the aerobic & anaerobic bacteria. Some claim that infauna are responsible for turning over the sandbed and causing the proper conditions for denitrification. Others claim that advection and the flow above the substrate causes small pressure differences within the sandbed which are responsible for nutrient transfer. Either way, the nitrates must make their way to the anaerobic bacteria, and the nitrate free water must make it's way out of the sandbed. I'm not saying that these processes do not take place. I'm saying that there are several debates as to how they take place. The two prevailing arguments are infauna and advection. Given those two arguments, I'm doubtful that a large particle substrate will be of benefit.
Maybe with a Jaubert system, where the denitrification process is soley dependant on bacteria and the molecular charge of nutrients, this substrate might be of benefit. There are still too many uncertainties with DSB's to say if this idea will work or not. So, let's look at both arguments and see how this substrate would interact.
If DSB's perform denitrification mainly due to infauna slowly turning the sandbed, it would be dependant on their ability to physically move the substrate. We're talking about micro brittle stars and polycheates. At least, those are some of the larger fauna performing these functions. I seriously doubt their ability to physically move or turn over such large particles. IF they only operate in the upper layers of the sandbed, then your idea might be beneficial. But nobody has absolutely proved how infauna operate in a DSB and to what extent when it comes to denitrification.
If DSB's function by advection and minute pressure differentials caused by variations in the substrate surface, then your idea may hold more validity. However, the amount of flow between a fine grain substrate is greatly reduced within the top inch of sand. When you propose using a larger sized particle, the flow of water between these particles will be greater to a greater depth. Regardless of the amount of bacteria these particles house, you still have to produce a low oxygen environment before denitrification can take place. If you have stronger flow through the top layers of substrate, you will have more oxygen in the top layers of substrate. Thus your bed must be deeper. Since you propose using this material only on the bottom, the chances of low oxygen water making it to your eheim layer are greater. This scenario might actually work, if DSB's operate based on advection.
Either way, I feel that there are too many unknowns. But your idea certainly would test some theories. So, by all means try it. Please, I ask, document it so that we can all see how it goes.
