What Prime does is turns the ammonia into ammonium, a substance that is harmless to fish, but treated the same as ammonia by the nitrifying bacteria. The reason you need this is because chloramine is a compound of chlorine & ammonia. The water treatments that say they treat for chlorine & chloramine while making no mention of ammonia will take care of what they state, and will work fine for smaller water changes in a mature tank, as the mature nitrifying bacteria will convert the ammonia to nitrite, then nitrate in a short amount of time.
For newly cycled tanks, or when doing larger water changes on a mature tank with water that is treated with chloramine it is a good idea to use a treatment that deals with ammonia. The bio filtration in a newly cycled tank may have a hard time dealing with the residual ammonia, exposing the fish to it for a longer amount of time. With larger water changes in a mature tank the same can occur. By converting it to ammonium the fish are unaffected, while giving your bio filtration a chance to catch up.
So... the component in products like Prime (sodium hydroxymethanesulfonate or a similar chemical) turn the ammonia into ammonium, which is then taken care of by your bio filtration. Nitrifying bacteria in a mature filter can double in 24 hours, so I doubt the ammonium is in the tank much longer than that. Test that use a single reagent (nessler test) will read ammonium as ammonia. Tests with two reagents (salicylate test) will not read ammonium as ammonia, thus avoiding a false positive.
