In the first stage of this study (Cancino Madariaga et al., 2011) we analyzed the effects of pressure and pH on three nanofiltration (NF) membranes, NF 200, NF 270 and NF 90, and on DSS-HR 98PP, a reverse osmosis (RO) membrane. The range used was based on the aquaculture pH level, i.e. between 5 and 7, and an ammonia concentration in terms of NH4 + . Transmembrane pressure (TMP) is the force that pushes the liquid through the membrane, result- ing in retention of part of the ammonia by the membrane, with the other fraction passing through the membrane. However, addi- tional effects on ammonia retention (AR) were discussed, finding that molecules with similar molecular weight to that of water (18 g/gmol) and NH4 + (17 g/gmol) have different retention rates due to the ionic force and the Donnan effect.