Hi wot_fan,
ADA AS has 2 stages where it releases massive amount of ammonium and organics. First is when it is first water logged, second is about 1 month later after the grain are totally soak and loosen.
As such, don't be deceived by the test reading saying 0 ammonia and 0 nitrite will be safe for shrimp. The best way to gauge this is by observing the critters's population and activity. When you see their population bloom and increased in activity, that means the first stage one completed. After awhile, their population will decrease tremendously. This signify the start of stage 2. During this stage 2, perform regular 20 to 40% water change. Once you see the population and activity of the critters bounce back, then it is the almost time for stage 2 to complete. After that, perform approximately 80% water change (drain the water until 4cm above the highest point of the substrate). Let the water run for 48 hours before adding in the shrimp.
If you want to speed up the leeching of ammonium and organics, use mild alkaline water. Approximately pH 7.6 or 4dKH.