The Seachem MultiTest: Ammonia 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.
The Seachem MultiTest: Ammonia 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 while using ammonia removal products such as Prime, Safe, AmGuard and any similar competing products. The other kits (salicylate or Nessler based) determine the total ammonia by raising the pH of the test solution to 12 or greater. At this high pH all ammonia removal products will breakdown and rerelease the ammonia, thus giving you a false ammonia reading.