hmmm, had the same on my 5 gallon tank, but I make my own water and was able to bring it down, and change the mix of minerals/salts I use.
My NO3 remains at about 30, seemingly no matter what i do, currently I read:
Ammonia 0, NO2 0, NO3 30, GH 100, KH 80, PH 7, Phosphate 5 PPM
Tank contains a betta, 4 cory's, and 3 snails, as well as live plants. The algae problem has been manageable, but I'd like to get the NO3 and Phosphate under control. Doing 35% water changes weekly.
Fish in seeming good health. Mr. Betta building one bubble nest after another.