This kit measures total (NH3 and NH4+) and free ammonia (NH3 only) down to less than 0.05 mg/L and is virtually interference free in marine and fresh water. Free ammonia is the toxic form of ammonia (vs. ionized Ammonia NH4+ which is non-toxic) and thus it is much more important to keep an eye on the level of free ammonia in your system. This kit may be used with freshwater or marine water. Unlike other kits on the market, the chemical basis for this test assures that interference from other substances is highly unlikely. MultiTest™ Ammonia performs over 75 tests and contains a reference sample for validation.