You need to make sure that the water is biologically ready before you let your shrimps in. The process of aquarium maturation is a process of building a small ecosystem. As in every environment, the nitrogen cycle takes place in aquarium. If you want to keep your animals healthy you need to maintain biological balance. A new tank lacks bacteria flora which regulates the level of ammonia. That is why you should wait 3-4 weeks for nitrobacteria to grow. Then the bacteria will be able to change ammonia into nitrite and finally into nitrate.