GFO will reduce phosphate, but should not affect nitrate. With a new tank you just have to be really careful about how much you feed since there is not a lot of biological consumption.If the chaeto doesn't end up reducing nitrates enough I may switch to a GFO reactor
One tip you can try is to do a 1 gal per day water change with TM bioactif salt. It has a little bit of carbon dosing in it, so that helps to get the denitrifying bacteria going. Just get 2 1 gallon buckets, make 1 gal of RODI, throw in 1/2 cup salt, mix for a min. Siphon out 1 gal with bucket 2 and dump in the new. Takes like 5 min a day.