I didn't read all of the thread, so not sure if someone mentioned it, but 0 nitrates doesn't mean you should slack on water changes.
Other organics need to be removed with water changes, not just nitrates, and water changes can also be necessary to replenish your buffers.
And I find it quite odd that your nitrates read 0 ppm without plants unless you constantly replace to carbon or have some kind of other nitrate reducing media.
The anaerobic environment needed to induce denitrifying bacteria is not common in enclosed freshwater systems (which is why people don't attempt to use deep sand beds in freshwater, as opposed to saltwater).
People that run denitrifiers on freshwater systems have specific filters set up to make an environment that will house denitrifying bacteria, and then those bacteria also have to be fed adequately.
It is possible if you have a lot of algae growth, that could do it, though I didn't see mention of that from your posts that I read.