What is the water change schedule?
It is good that they stay that low. I consider 20ppm to be the roof of safe nitrate, others will say 40ppm or more. The exact number is arbitrary but lower is always better. The important thing is that it stays low long term. Even a seemingly good water change schedule can be not quite enough and allow it to go up slowly over time.
Water changes and live plants are the two best ways to get rid of nitrate (and all the other bad things that build up over time). The ways nitrate get into a tank are: water changes (you always have some starting concentration), food, and the livestock itself (even without food they will give off some waste).