First, re-check: testing for nitrate can be tricky, follow the instructions. You do need to shake the tube for 1 min to get reasonable results.
I do keep one of my tank at 0 - the plants used it all up would be one reason.
Another reason would be un-cycled tank, do check for ammonia and nitrite.
If everything is good, overfeed the tank abit for 2-3 days and test again. You might also look into fertilization - tons of posts on the subject here.
Courtesy * Integrity * Perseverance * Indominable Spirit * and Self Control
Tenets of TKD