In recirculating aquaculture systems (RAS), high degrees of recirculation are beneficial from an environmental perspective, but they also imply high risks of accumulation of unwanted substances in the water loop. The fundamental features and the recent developments of nanofiltration (NF) potentially make this technology attractive for applications in intensive RAS. Indeed, NF membranes can retain dissolved contaminants of emerging concern, off-flavor molecules, and inorganic species (e.g nitrate and nitrite), and thus they can in principle be applied in RAS in order to remove such substances. On the other hand, the water productivity and the need for pre-treatments remain causes of concern for the integration of NF in RAS. Moreover, NF membranes partially retain all the dissolved ions and the non-toxic organic matter, thus altering the composition of the water in a way that might not be beneficial for the farmed fishes. For this reason, it is hard to foresee the use of NF as an alternative for the present biological treatments. On the contrary, NF technologies appear highly suitable to treat side-streams of the fish tank effluents after the biological treatments to prevent unwanted substances to accumulate and reaching critical concentrations in the water loop. Furthermore, the ongoing innovations in NF technologies are expected to facilitate their integration in RAS.