When you first set up an aquarium it is devoid of any bacteria. So when you start putting fish, or in your case, shrimp into the tank there is no bacteria to break down waste and absorb the ammonia and nitrite that develops from this. With time the bacteria will begin to develop and bring ammonia and nitrite levels down to safe levels. Once a bacteria table is established the tank is consider to be cycled. On average this can take up to a month to happen.