Do you have a TDS
I use saltyshrimp and have never had an issue with ammonia changing.
Use a TDS
meter and slowly add saltyshrimp into the tank until you get right under the desired TDS
for the type of shrimp you have- neos are normally 150-200 (i keep mine around 200).
Im wondering if you inadvertently added to much and it killed something in the tank.