Nitrates are a form of nitrogen. Also, nitrite and ammonia are a form of nitrogen as well. So if you have a planted tank, the plants will also consume any ammonia that is present, as well and nitrite and nitrate. But what you dose won't be the more toxic forms such as ammonia or nitrite, but nitrate instead, as it's much less toxic to fish. And you'll just be keeping the levels at 15-20ppm, so there's no harm to the tank whatsoever.