You coiuld have nitrates already in the water from your source wate. Test the source water to see if you do. If you dont test the parameters of your source water this will skew the results from your test.
Use your regular dechlor and aerate it in a bucket for 24 hours before testing tap water. It changes quite a bit compared to right out of the tap.Will do this when I get home today.
bacteria need oxygen and food to survive