There are heterotrophic and autotrophic denitrifying bacteria .. if running in a carbon biopellet aerobic format my understanding is that you are encouraging heterotrophs in large volumes which need to be used in conjunction with a protein skimmer .. if you opt for a reductor or anaerobic high volume very finely porous nitrate filter a low carbon dosing regime might improve efficiency and as the nitrates are released as n2 and the bacterial colony is fairly stable a protein skimmer isn't necessary .. if using highly porous material I would say 20% should be replaced monthly or every 2months