This kit measures total nh3 and nh4 and free ammonia nh3 only down to 005 mgl and is virtually interference free in marine and fresh waterr 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 systemr this kit is based on the same gas exchange technology that is used in the ammonia alert and thus is the only kit on the market that can read levels of free ammonia whil