Either you do not feed your culture enough, or you feed too much. If your water clears between 2 feedings, it is likely that you are in the first situation. In this case, you just need to gradually increase the amount of food delivered to the crop. If the water does not clear between 2 feeds, the amount of food delivered may be too large compared to the number of daphnids present. In this case, all microalgae can not be consumed in 24-48H. As a result, the ammonia level is increased, resulting in a decrease in the population size of daphnia. It is necessary to act quickly and to change a large volume of the culture (min 50%) and / or to decrease the quantity of food delivered.