It depends on the shrimp. Some shrimp (like Cherry Reds) will breed readily if the tank conditions are good and the food supply is good; the females carry the fertlized eggs around under their tails and the babies hatch as miniature adults. Other shrimp (like Amanos, some ghosts, Opae-ula) as adults can live in freshwater but their newborns (larvae stage) need saltwater within the first week of hatching to morph into mini-adults when they can be transferred back to freshwater (some have to be in saltwater longer than others). You can read about various (not all though) shrimp types at www.petshrimp.com to get an idea of what each type of shrimp's needs are, etc.