Phisher,
Can you give a better definition of "Grass Shrimp". AM is referring to the fresh water shrimp (called grass and glass shrimp but there are brackish water species going by the same names) but I have always called shore shrimp, grass shrimp. Do these come from full fresh water or from very brackish water? If you are unsure, try putting them in full salt (you may lose a buck in the attempt). If they die, forget it, they are not a good food but if they live, they are what most of us use. They tend to do better in lower salinity but I just use my regular salt mix and have few losses since they live in water that changes composition regularly.