Urbaneks. Seriously reconsider the biopellets addition ...
A slightly elevated NO3 level is something you dont need to be concerned about, unless it continues to increase above 10-15ppm, in fact it can/will help you more efficiently lower PO4 levels naturally through small carbon doses.
*Redfield Ratio
Ill take some of those NO3s, versus having the added work/risk of dosing KNO3/NaNO3 in order to further bring down PO4 in the bacterioplankton based filtration utilized on my systems ...
Nitrates at 1-3, even upwards of 10 isnt anything to be concerned about. Itd be the equivalent of taking out all of your substrate and inserting Starboard because you're getting a slight SiO2 reading. You'll be assuming a great deal of risk to 'fix' something that isn't broken ...
I'd highly suggest, instead of biopellets, to initiate a non-aggressive carbon dosing (vinegar/vodka solution, not just vodka) regimen. Also, consider slightly decreasing your GFO amounts, as severely limited PO4s may very well be inhibiting the consumption of NO3s regarding the bacteria you are dosing !
Shouldn't mess with a product that has many negative attributes, including the ability to drastically/rapidly change your chemistry, relatively uncontrollably, when there's really no need whatsoever.
"Drops" of vinegar will undoubtedly take care of your supposed problem, barring an extreme lack of PO4.