- Recirculating aquaculture system (RAS) effluent has a high concentration of nitrogen, which can be a valuable asset as fertilizer for agricultural use in the vicinity of the RAS. A novel fully-automated experimental set-up of 24 lysimeter-plots was used to determine the optimal irrigation rate for cucumber plants fertigated with RAS effluents of three different nitrogen concentrations. Three RASs were stocked with 7, 14 and 21 kg of barramundi ( Lates Calcarifer ), which were fed 1.75% of their body mass daily. Then, 10% of the water in each RAS was exchanged daily, resulting in three nitrate–N concentrations of approximately 30, 60 and 90 mg/L. A synthetically fertilized control treatment was kept at 60 mg/L nitrate–N, and the other essential plant nutrients were kept at the same concentration for all treatments. The water from these four sources was then used to irrigate six plots per nitrate–N concentration treatment, at a rate of 1–6 times the amount transpired, which was measured in an automated fashion by the experimental set-up. Two yield response to nitrogen fertigation models fit well to the measured data, and although they led to different conclusions in terms of optimal fertigation can be a useful tool for decision support. The nitrogen use efficiency dropped with increased fertigation, from about 80% to 55%, and more fertigation led to an exponential increase in drainage and gaseous emissions of nitrogen. The observed data can be used to optimize irrigation for each nitrogen concentration, but the values are highly dependent on climate, plant type, and root zone characteristics, demonstrating the need for more inclusive modelling. Determination of how to optimally make use of RAS effluent water and the comparison with synthetic fertilizer can lead to the development of simplified protocols concerning integrated aquaculture agriculture systems, making it accessible to a wider range of growers.