The bacteria that would do nitrate removal are not typical anaerobes, they are what are called facultative anaerobic bacteria or facultative aerobic bacteria. They can function in either aerobic or anaerobic environments. When oxygen is present they use that. But if the oxygen is depleted from the water, they can use nitrate instead. Bear in mind nitrate is NO2. The bacteria can strip out the oxygen and that leaves nitrogen gas in some form which then exits the tank in that form. While there is always some level of denitrification happening in a mature biofilm, it is usually too little to make a big difference in a tankl's average nitrate levels, However, in a planted substrate or massive ppi controlled media, there will be a much greater level of potential denitrification.
You do need need hugely reduced flow rates that one would typically see in a dedicated denitrifying filter. However, I do not run anything close to 10x turnover rates in my mattenfilters or Poret foam cubes. The only time I ever monitor for nitrates is when I have to run a dry/rainy season to encourage spawning by some of my plecos. They do not do well, especially the youngsters with nitrate. With the above Poret filtration established, I almost never see nitrate, even when I am reducing water changes. I need the TDS and the pH to rise over time (about 8-10 weeks) but not the nitrate.
The key to understanding the above is that water can follow a myriad of pathways through massive media. In some places the water has the oxygen used up and the facultative bacteria will do their thing. But in other routes through the media the oxygen will not be used up and there is no denitrrification happening. The last thing we want of for the water coming out of a filter, especially a canister, is for it to be low or devoid of oxygen. The nitrifying bacteria need oxygen to process ammonia and nitrite. So do the fish and other critters. Even plants use oxygen at night.
You do need need hugely reduced flow rates that one would typically see in a dedicated denitrifying filter. However, I do not run anything close to 10x turnover rates in my mattenfilters or Poret foam cubes. The only time I ever monitor for nitrates is when I have to run a dry/rainy season to encourage spawning by some of my plecos. They do not do well, especially the youngsters with nitrate. With the above Poret filtration established, I almost never see nitrate, even when I am reducing water changes. I need the TDS and the pH to rise over time (about 8-10 weeks) but not the nitrate.
The key to understanding the above is that water can follow a myriad of pathways through massive media. In some places the water has the oxygen used up and the facultative bacteria will do their thing. But in other routes through the media the oxygen will not be used up and there is no denitrrification happening. The last thing we want of for the water coming out of a filter, especially a canister, is for it to be low or devoid of oxygen. The nitrifying bacteria need oxygen to process ammonia and nitrite. So do the fish and other critters. Even plants use oxygen at night.
