The bacteria that convert nitrate to nitrogen gas are anaerobic bacteria that rob the oxygen from the nitrate to convert it to nitrogen. Anaerobic digestion is not a desired bacterial process in the ponds and tanks as it is also responsible for the production of hydrogen sulfide gas which is highly toxic and the production of methane. Some media that is highly porous allows water with nitrates to pass through and bacteria near the surface use the available oxygen while bacteria deeper inside need the oxygen from the nitrates to survive, these are best installed in a shower type filter.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard