That is exactly why it does work, because water is not moving through it. If water was moving through it, it would be nothing but an undergravel filter, addressing only the first half of the nitrogen cycle: ammonia --> nitrate, which is an oxidation process.
If holes are drilled in a media (sponge rock for instance) to allow water to easily flow through, you end up with an undergravel filter again...it is unproductive in terms of denitrification, becasue the oxygen level will not drop sufficiently during the traverse. Hence the issue of 'at the precise speed"
FYI, bottled bacteria are heterotrophic bacteria cultures. Heterotrophic bacteria, are not responsible for any part of the 'nitrogen cycle.' ammonia --> nitrogen gas. Heterotrophic bacteria produce the "ammonia" (organic --> inorganic) that subsequently is processed to nitrogen gas by the "nitrogen cycle," performed by autotrophic bacteria. Bottled bacteria is snake oil, and a waste of time. There was a great deal of "playing" with these concepts back in the late 70s and early 1980s: the black box denitrators. All of which promoted heterotrophic bacteria, which biologically speaking, is the wrong approach.
