Because fish are constantlly producing ammonia and nitrite, there is always a trace, but the beneficial bacteria in filters and every surface in the tank, almost immediately convert it to "less toxic" nitrate.
Most natural unpolluted waters are very low in nitrate.
Before I retired, part of my job was to test Lake Michigan water daily, nitrate barely ever exceeded 1ppm.
And I have tested waters here in Panama where my fish and inverts (like freshwater shrimp) come from, and I have yet to get readable readings with my API kit of more than 1 or 2 ppm.
Most natural unpolluted waters are very low in nitrate.
Before I retired, part of my job was to test Lake Michigan water daily, nitrate barely ever exceeded 1ppm.
And I have tested waters here in Panama where my fish and inverts (like freshwater shrimp) come from, and I have yet to get readable readings with my API kit of more than 1 or 2 ppm.