Pasfur I've found 2 kinds of Denitrifiers. The coil one just pumps water through the coil ate a very slow rate, then the water goes up a tube filled with biomaterial and then drips in the tank.
Now the reason for the die off is the following as I have read and understoon in another forum.
The very slow flow of the water is because of the bacteria developed in those biomaterial can't survive when there is regular or even slow flow.
Those bacteria are the same or do the same work as the bacteria in a deep sand bed. Having a denitrifier stops bacteria from being developed in the sand bed. So when you remove the denitrifier you have a sand bed that doesn't have enough bacteria to handle the load of nitrates.
Well I don't know if I have understood correctly. Based on your experience is the above senario plausible or could it just be nonsence?