What does it mean?
TAN stands for Total Ammonia Nitrogen or just Total Ammonia (TA). Total ammonia is a measure of the concentration of free ammonia (NH3) and ammonium (NH4+) in water. The term is used when it is important to identify the type ammonia being measured as the term ammonia may refer to free ammmonia, ammonium or total ammonia. Ammonia test kits vary as to which forms of ammonia they test for. This is important because free ammonia is significantly more toxic than ammonium.
See Ammonia for more details.