The reading of NH4 is in PPB and not in PPM, that is why 15~20 is considered as acceptable. Also, the reading is not real because it dose not depend on a real sensor. It depends on some mathematical equations which are not very accurate sometimes.
Seneye is a great device but it needs a lot of development to include several sensors to give a full idea about the current water condition and its progress, till that happens you have to do those tests manually.