Hi, I also have the AP master test kit and never get a zero reading for
ammonia or nitrates. I've taken a sample to a LFS and they have used a
different kit and told me it was clear. I always get the minimum
reading. I've had this discussion with others on this board who also
have the same problem. Take a sample to an LFS who uses a different
kit and see what they say. I have to say that the dip strips that I've
used haven't been at all impressive. I still use the AP test as i like
the ease of testing. I just know to expect these reading.

Hope this helps
Mellie