The API nitrate test is tricky. If you don't shake the 2nd bottle enough it will read really low or even 0. I usually give it a good shake (30+ seconds), set it aside while I am testing the ammonia and nitrite and then shake it good again, get the test set up with the water and the first chemical (bottle #1 - don't forget to shake this up some after you've added it to the water) and then shake bottle #2 again for 15-30 seconds. then you have to shake the test tube for several (30 +) seconds, and then set it aside for a little bit. You should see good color developing when you are shaking the tube after you add the second chemical. if you don't shake the holy snot out of bottle #2 you will not get accurate results. Period