Ammonia is a toxic metabolite of protein catabolism that can limit growth and health of aquatic animals. This study investigated the effect of dietary protein level on the average total ammonia (TAN) and free ammonia nitrogen (FAN) concentrations in a serial-use raceway used to farm South African abalone, Haliotis midae Linneaus. Three isoenergetic diets contained 33% (P33), 26% (P26), and 22% (P22) protein. Biomass of abalone was 7.6 ± 0.1 kg/300-L tank (45–55 g/abalone). TAN and FAN concentrations were significantly correlated with dietary protein (P) (P < 0.0001) and flow index (FI; as measured by liters per hour per kilogram; P < 0.0001), and could be estimated using the models TAN = 9.73 P – 110.3 log (FI), and FAN = 0.132 P – 1.10 log (FI). Mean FAN concentration in the P22 and P26 treatments was 67% and 41% lower, respectively, than in tanks fed the P33 diet. Because this species can grow well on low-protein diets, it is hypothesized that a reduction in percentage protein will improve the carrying capacity of serial-use systems. The models estimate to what extent discharge of nitrogen could be reduced through lowering the protein level in the formulated diet.