Fatty acid source in tilapia (Oreochromis niloticus) feeding can affect stress resistance and thus fish welfare. We aimed to test the hypothesis that diets based on different sources of fatty acids in tilapia reared in a Recirculating Aquaculture System (RAS), affect fish welfare as measured by different stress and immunity indicators. A total of twenty four male tilapia individuals were fed Fish Oil (FO), SunFlower Oil (SFO), Linseed Oil (LO) or High-Oleic SunFlower Oil (HO--SFO) as a fatty acid source. Water quality, cortisol and liver transcript expression (genes PRLRa, KLR, HSP70, IGFI, IGFII, GHR) were measured. No differences in growth performance were shown when changing fatty acid source. Water quality was improved in the tanks where fish were fed on plant oil [Electrical Conductivity (EC25) was lower (p<0.023)]; concentration of chloride (Cl-,p<0.001), calcium (Ca2+,p<0.0001) and sodium (Na+, p<0.012) ions were lower when HO-SFO oil was used, suggesting better nutrient absorption. Also, KLR, HSP70 and PRLRa genes were significantly inhibited and IGFI, IGFII and GHR significantly over-expressed in individuals fed vegetal oils when compared to FO, indicating an osmoregulatory stress in fish fed the latter. We conclude that growth is not affected by substitution of fish oil by vegetal oils, and that the latter lowers stress, improving fish welfare.