Synergistic and Antagonistic Effects of Combined Dietary Iron and Potassium on Lettuce Growth Quality and Fish Physiological Responses in Aquaponics

Aquaponics is a resource-efficient agricultural system, yet its overall productivity is frequently constrained by micro- and macronutrient deficiencies, particularly iron (Fe) and potassium (K). Currently, the efficacy of combined dietary Fe and K supplementation in optimizing nutrient management in these systems remains unclear. Therefore, a 60-day feeding trial was conducted to evaluate the effects of four dietary Fe and K levels—CK (basal diet without added Fe or K), T1 (Fe 0.1 g/kg + K 2.5 g/kg), T2 (Fe 0.2 g/kg + K 5.0 g/kg), and T3 (Fe 0.3 g/kg + K 7.5 g/kg)—on the growth and quality of lettuce (Lactuca sativa) and the physiological responses of crucian carp (Carassius auratus). The results demonstrated that the T2 treatment was suitable for enhancing system productivity. Compared with the CK group, the lettuce plant height, biomass, and net photosynthetic rate in the T2 group exhibited marked increases of 25.3%, 16.0%, and 26.4%, respectively. Furthermore, the vitamin C content increased by 52.2%, while the nitrate content notably declined by 32.2%. Plant nutrient analysis revealed that the combined Fe and K supplementation markedly promoted the foliar uptake of P, Mn, and Zn (peaking in the T2 group), whereas Cu and S contents increased linearly with the supplementation dose, reaching its maximum in the T3 group. Regarding fish health, the weight gain rate (WGR) of crucian carp in the T2 group peaked at 41.0%, and the feed conversion ratio (FCR) improved to 1.76. Additionally, the dietary supplementation maintained the stability of water quality parameters of the recirculating system. These findings indicate that a combined dietary inclusion of 0.2 g/kg Fe and 5.0 g/kg K can simultaneously enhance the yield and quality of both plants and fish. This approach provides a novel optimization strategy for mitigating acute water quality shocks, while also highlighting the inherent boundaries of competitive ion antagonism in aquaponic nutrient management.