Farmed fish high in n-6 PUFA may undermine fish nutritional expectations and intake recommendations for n-3 PUFA requirements and exacerbate rather than improve already high n-6/n-3 PUFA diets. Dietary contribution of fish fortification by linseed-based n-3 PUFA was evaluated. Mango tilapia (12 months old) with high n-6 PUFA (21.8 FA%, n-6/n-3 ratio 4.6:1) were fed standard/control (T(C)) or linseed-supplemented (5%, T(5%); 7%, T(7%)) feed for 61 days regular-growth and 120 days stock-growth (to 650 g). Compared to T(C), n-3 PUFA increased in T(5%) 46% and T(7%) 58%; ALA in T(5%) increased 100% and T(7%) 167%; EPA+DHA in T(5%) increased 14% and T(7%) 23% (p < 0.05); n-6 PUFA/LCPUFA were unchanged. T(7%) EPA+DHA 168 mg/100 g of raw fillet is comparable to current American intake and Dietary Reference Intakes; controlled cooking preserved approximately 90% EPA+DHA. n-6/n-3 ratios decreased 16-38% in total PUFA to 2.3:1 and in LCPUFA to 0.61:1. Linseed supplementation could improve tilapia n-3 PUFA/LCPUFA, ameliorating n-3 PUFA scarcity and unexpectedly high fish n-6 PUFA content, potentially making a significant nutritional contribution.