A couple of things - keep the water temps relatively low, and limit the number and size of fish in each bag:
To quote:
"The metabolic rate of fish will double for each 18 degree F increase in temperatures and be reduced by half for each 18 degree F decrease in temperature. A reduced metabolic rate will decrease the oxygen consumption, ammonia production and carbon dioxide production. Therefore, it is essential to transport fish as low temperatures."*
"The easiest way to reduce toxic ammonia buildup in transport water is to lower the temperature of the transport water and to stop feeding several days before transporting."*
Slowed ammonia and carbon dioxide production both help to prevent ammonia buildup.
Randy or some of the Chem forum regulars may have more ideas, but - given the level of ammonia needed for acute toxicity in fish** - I doubt more than that is needed for properly stocked transport bags.
*Source:
Reviewed by Robert A. Pierce IIExtension Fisheries and Wildlife SpecialistReviewed by David BruneProfessor, Plant Sciences and TechnologyL. SwannIllinois-Indiana Sea Grant Program, Purdue University
extension.missouri.edu
**Source:
In the context of trying to understand why so many people believe Seachem Prime was useful for them in an elevated ammonia situation, against the data in the link below about Prime potentially doing nothing to free ammonia in seawater, the question arises, why...
www.reef2reef.com