Nitrite inhibits the blood's ability to carry oxygen. I agree with Coler, in a cycled tank your nitrAte shouldn't be zero, your nitrIte should. If you're using an AP Freshwater Master Kit you could be doing the nitrate test wrong, causing it not to measure. You have to follow the directions on the nitrate test to the letter (shaking the bottles and test tube).
If your Ammonia and nitrite are zero, it could be a case of not enough water turnover at the surface. The surface of the water is where gaseous exchange takes place. You can use an air stone as suggested or powerhead pointed toward the surface to keep the water circulating to the surface continually.
Mark