I was running nitrates of 40+ ppm following the cycle of my tank despite water changes. I started vodka dosing and switched to NP-pellets because of the hassle of dosing. Within a 3 week time frame my nitrates are undetectable with a Salifert test kit.
As far as phosphates go, from what I've read biopellets have very little effect. GFO, avoiding overfeeding and a refugium with macro algae should be used to control phosphates.