Many ferts will add nitrogen, though not necessarily all. They may also add potassium or phosphorus. It will be clearly stated on the label.
Most people here recommend keeping a nitrate in the 10ppm range or lower, though 20ppm is still considered safe. You'll never have them to zero if you have fish in there. At these levels, the nitrates are essentially harmless. OTOH, you should change some of the water on regular basis regardless of how low the nitrates are, as other solids will accumulate as well.