Feeding Time Optimization Enhances Aquaponic Performance: Growth, Water Quality, and Nutrient Removal in Systems Integrating Cyprinus carpio and Lactuca sativa

Feeding time is a critical but understudied factor influencing nutrient dynamics and overall productivity in aquaponic systems. This study examined the effects of two feeding schedules on growth performance of common carp (Cyprinus carpio L.), hydrochemical parameters, and the growth of lettuce (Lactuca sativa) cultivated in an integrated aquaponic system. Two 60-day trials were conducted over consecutive years under identical greenhouse conditions. Carp were fed either in the morning and early afternoon (T1: 08:00, 11:00, 14:00) or later in the day (T2: 11:00, 14:00, 17:00). Hydrochemical indicators, including dissolved oxygen, turbidity, ammonium ions (NH4+), and nitrates (NO3−), were continuously monitored through online measurement. Carp reared under T2 displayed significantly higher specific growth rate, final body mass, and improved feed conversion ratio (p < 0.05). The T2 variant also showed higher dissolved oxygen levels and lower turbidity compared to T1, indicating enhanced system stability. Although NH4+ concentrations were higher and NO3− levels lower in T2, these differences did not compromise water quality due to efficient plant nutrient uptake. Lettuce grown under T2 exhibited greater stem and root development and higher biomass accumulation, suggesting improved nitrogen utilization linked to the NH4+/NO3− ratio and enhanced root oxygenation. Overall, aligning feeding time with fish circadian rhythms improved fish performance, plant growth, and nutrient cycling efficiency. These findings demonstrate that feeding schedule is a key management factor capable of enhancing sustainability and productivity in aquaponic systems.