I would say your tank is cycled. Congratulations.
From my own experience, test strips for Nitrites and Nitrates are not so reliable (the quantity measured might be inexact) but I doubt you would get a false negative with them. The validity of Nitrite and
nitrate strip tests (they measure what they are supposed to measure) has been fairly good for me.
That usually means that a test strip measuring zero nitrites or nitrates is likely telling the truth. Problems begin when they actually pick some readings. They usually exaggerate the actual
PPM in the tank. That's why I use them as screening test. I use liquid reagents test for reliable readings.
Pepe
Santo Domingo