Actually, the eggs hatch in freshwater. After they do, the larvae will then need to be moved to saltwater, where they swim about until they reach a postlarval state. Then they will need to be acclimated to freshwater.
I've actually been working on this myself with the amano and a couple of other species of shrimp. I haven't had much luck so far, but things are looking promising with the latest batch of larvae.
Nevertheless, there's always a slight chance that one of the small ones gets through. I'm pretty sure that mine got one or two more than when I bought them a year ago.
Not only do the young need brackish to full salt water salenity, they need special food to mature (marine green water). They can not eat flake food, or non floating algae, until they mature.