Not sure, but the Pro Ammonia Detox probably contains an ingredient that binds with the ammonia molecule, thereby rendering it harmless to fish and inverts because it has been effectively "removed" from the system. Because the ammonia is in reality only bound to another molecule, it can be "unbound" as well. Nessler's reagent may do that and show a false positive. I suspect Nessler's reagent probably isn't something the hobbyist is likely to encounter. It is HgI4K2. Test kits do contain some nasty things, but most kits I believe use salicylate, not Nessler's reagent. When in doubt check the box. If it says "total ammonia" it could be either salicylate or Nessler's reagent. Salicylate seems to be more common as it is less toxic than the mercury in Nessler's reagent. Some people are sensitive to salicylate, but most foods contain it, as does aspirin.