Which ones disappear, the Ghost shrimp or the Amanos?
How are you acclimating them to your tank/water, when you buy them?
Either will be alright in FW. Fish like to eat them, they are especially vulnerable after molting, and can be sensitive to changes in water parameters/ poor water conditions, and could be in poor condition/sensitive from being transported, kept in ammonia polluted shipping bags.
There are some differences in the kinds of shrimps which are called feeders, some are Grass Shrimp. Lots of names are used by importers and breeders. But if they are called Ghost shrimp and have a sharp angle on the back of the abdomen usually are Ghost shrimp.