http://www.seachem.com/support/FAQs/Prime.html
Q:I tested my tap water after using Prime and came up with an ammonia reading. Is this because of chloramine? Could you explain how this works in removing chloramine?
A: Prime works by removing chlorine from the water and then binds with ammonia until it can be consumed by your biological filtration (chloramine minus chlorine = ammonia). The bond is not reversible and ammonia is still available for your bacteria to consume. Prime will not halt your cycling process.
I am going to assume that you were using a liquid based reagent test kit (Nessler based, silica). Any type of reducing agent or ammonia binder (dechlorinators, etc) will give you a false positive. You can avoid this by using our Multitest Ammonia kit (not affected by reducing agents) or you can wait to test,
Prime dissipates from your system within 24 hours.