Seachem offers Matrix as a good "all-in-one" bio-media solution in that it performs great with flow rates commonly found in sumps/canisters. It is able to maintain good de-nitrification regardless of flow rate by virtue of its size. The larger the individual "pebbles" the more internal room for anaerobic bacteria and larger buffer zone to delineate the anoxic zone within the pebble from the oxygen rich zone at the surface.
Pond Matrix further carries this principle with much larger rock sizes designed for much larger applications and much higher flow rates.
Their De-Nitrate product is the exact same material, but a much smaller size, almost gravel like. That product is specifically targeted at the de-nitrification process and is recommended that the flow rate be somewhere around 25-50 gph
Again the reason is related to the size of the individual rocks. The smaller rock will have a much smaller "buffer" zone for the anoxic area within.
The reason the smaller ones are used for De-Nitrate with a slow flow rate is that with a smaller rock the overall surface area, external and internal, is increased exponentially and it helps to compensate for the limitations of achieving bacterial de-nitrification in an oxygen rich closed system.
what the slow flow helps accomplish is allowing enough time for the aerobic bacteria to consume the majority of the oxygen in the water as it flows through the media, thereby reducing the size of the requisite buffer zone that the anoxic bacteria requires. That is why if utilizing such a bio-de-nitrification filter, it should be at the end of the line, but the effluent should be oxygenated and possibly ph adjusted prior to returning to the tank.