Nitrates are never too low! If you don't have any it's good! On a liquid test the max number of safe nitrates should be 20. If it's under that it's great! If your filter's working and it's cycled then you'll always be getting more nitrates. You just remove them with water changes. I doubt you'll ever have zero nitrates, so you don't have to worry about "too low". Keep nitrates under 20 and ammonia/nitrites at zero if possible!