Recirculation aquaculture systems (RAS) need to control ammonia level because the high toxicity on fish. For this, nanofiltration membrane has been proposed in the first part of our study to separate ammonia from water. We discussed different membranes as well as transmembrane pressure (TMP) and pH to achieve the best results for ammonia retention (AR). In this second part we analyze the influence of the ammonia concentration on AR to obtain higher concentrated volume that can be separated of the system and treated later. A model solution in a non-steady state was used, where the ammonia concentration increase over the time. Since water has similar molecular weight that ammonium ions, the mechanisms for the rejection of ammonia are based on the repulsive electrochemical forces on the active layer. A RO membrane DSS-HR98PP and three NF membranes, NF90, NF200 and NF270 were tested over a range of 0-11.45mg/L under optimal TMP of 16bar for NF membranes and 24.5bar for the RO membrane. Projecting the results to the RAS, NF270 is the most efficient for ammonia retention, as both flux and AR influence the engineering design concept. For example, treating 2000L/h with an ammonia concentration of 7mg/L, removal is 17.5g/m2day with a membrane area of 13m2 and TMP of 16bar.