Do you have plenty of
live rock? Do you have a heavy fish load? Are you using tap water, or
RO/DI for your source water? Is the tank relatively new?
I've found through keeping several nano tanks that as the tank matures and stabilizes (as I'm sure you know, this can take many months), the nitrate will eventually hit 0, or close to it, and stay there. If you have ammonia or nitrite, there is something going on in the tank like maybe overfeeding or a mini cycle. What is your maintenance schedule like?