Manipulation of Nile tilapia (Oreochromis niloticus) diets with potassium diformate enhances lettuce (Lactuca sativa L.) yield in aquaponics

Abstract The sub-optimal concentration of potassium (K) in coupled aquaponic systems hinders productivity and usually necessitates the use of chemical fertilizers, which compromises the sustainability of the system. The dietary inclusion of potassium diformate (KDF) may alleviate this problem by enriching the fish water with K; however, this has never been tested in aquaponics. In the present study, three dietary KDF rates (12, 18, and 24 g kg −1 ; KDF12, KDF18, and KDF24, respectively) were tested in a coupled aquaponics system, with Nile tilapia fed basal diet lacking KDF supplementation serving as the control. In this non-supplemented group, the K level in the water was measured as 3.0 ± 0.2 mg L −1 , representing the sub-optimal conditions inherent in such systems. Several growth parameters of fish and lettuce were monitored during the 45-day growth period and at the final harvest. Additionally, the fatty acid (FA) composition for fish was assessed, as well as an indicator of the total chlorophyll content (SPAD index) and the color characteristics of lettuce. At the end of the study, the KDF18 diet significantly improved growth outcomes for both fish and lettuce, followed by the KDF24 diet. Moreover, fish fed the KDF18 diet had a 25% higher weight gain (WG) compared to the control group. Lettuce’s final fresh weight increased by 126% in the KDF18 group compared with the control. Differential responses in the FA profiles of fish muscle were recorded. While Ʃ Saturated fatty acid (SAFA) levels increased with the dietary KDF levels, this was accompanied by a significant increase in eicosapentaenoic acid (EPA) + docosahexaenoic acid (DHA) and total n-3 polyunsaturated fatty acid (PUFA) levels, particularly in the KDF24 group. We have determined that KDF, especially at a rate of 18 g kg −1 , appears to be a promising alternative that could address certain aquaponics bottlenecks posed by sub-optimal K levels, promoting Nile tilapia growth and crop productivity.