Yep, it's a part of the nitrogen cycle. The good thing is, plants will use this as a nutrient and depending on your plant load, it'll probably never cause a problem. In a planted tank, you have to keep a minimum of 10ppm nitrate at all times.
Quite the conumdrum eh? In a regular
FW tank, you are fighting to keep nitrate levels down. In a planted tank, you have to keep nitrate levels up.