Optimal water quality is considered as being a restriction for marine copepod cultures for live feed. There is a lack of knowledge on the water‐quality conditions in copepod cultures and the effect on copepods. Few studies have investigated the effect of ammonia on copepods, and fewer reports No Observed Effect Concentrations (NOEC) and Lowest Observed Effect Concentrations (LOEC), which provides safety levels before cultures are affected. This study investigates the tolerance of Acartia tonsa nauplii and adults to ammonia, using mortality as the endpoint after 24, 48 and 72 h of exposure. Nauplii were exposed to levels from 0 to 5127 μg NH3 L−1 and adults to levels from 0 to 8481 μg NH3 L−1. Nauplii NOEC was 30 μg NH3 L−1 and LOEC was 81 μg NH3 L−1. Adult NOEC was 477 μg NH3 L−1 and LOEC was 1789 μg NH3 L−1. 50% Lethal Concentrations (LC50) for nauplii of 48 and 72 h was 1257 and 220 μg NH3 L−1. LC50 for adults was 2370 (24 h), 972 (48 h) and 770 (72 h). Combining NOEC with excretion rates of NH4/NH3 a model was developed to calculate densities in batch cultures. We recommend that for batch cultures of A. tonsa, NH3 is kept below NOEC for nauplii and that levels of NH3 together with pH are monitored weekly.