In this study, we assessed the effects of dietary Fe level on the growth performance, nutritional status and health condition of mirror carp (Cyprinus carpio var. specularis) (12.59 ± 1.73 g), production and quality of lettuce (Lactuca sativa var. ramosa Hort) in recirculating aquaponic system for a 60-day feeding trial. Five semi-purified diets with ferrous sulfate as the iron source were used in the feeding trail, which were: T1 (53.81 mg Fe/kg); T2 (118.83 mg Fe/kg); T3 (209.24 mg Fe/kg); T4 (328.44 mg Fe/kg); T5 (498.74 mg Fe/kg). The findings indicated the following: (1) The growth results showed that compare to other groups, the weight gain of mirror carp in Group T2 was the highest, and the feed conversion ratio was the lowest (P < 0.05). (2) The body composition results showed that muscle crude fat and crude protein of the mirror carp significantly increase in the T2 group compared to the T1 group (P < 0.05). Additionally, the C20:5n-3 levels in both the T2 and T3 groups were significantly higher than that of T1 group (P < 0.05). (3) Group T2 exhibited the highest liver superoxide dismutase activity, with significantly elevated catalase activity compared to the other groups (P < 0.05), and the liver malondialdehyde content in group T2 was significantly lower than the T4 and T5 groups (P < 0.05). (4) The morphological measurements of intestinal tissues in mirror carp showed that compared to Group T1 and Group T5, Group T2 exhibited the highest villus length and muscular layer thickness (P < 0.05). (5) The lowest yield of lettuce is in the T1group (3.35 kg/m2/month), and the highest yield (3.96 kg/m2/month) is in the T5 group, along with the highest chlorophyll and soluble sugar content (P < 0.05). (6) The T5 group had the highest nitrogen utilization efficiency (69.44%), while the T1 group had the lowest nitrogen utilization efficiency (58.37%). In addition, the different dietary iron levels did not adversely affect the water quality of the aquaponic systems. Based on the line regression analysis of weight gain and feed conversion ratio with feed iron content, it was found that the most suitable iron content in feed is 118.22–120.44 mg/kg, and the production and quality of lettuce will increase with the increase of feed iron content. According to the results of fish and vegetable growth, as well as nitrogen utilization efficiency, it is suggested that the dietary iron level in the feed of juvenile mirror carp utilized within aquaponic systems should ideally fall within the range of 120–200 mg/kg.