Efficient nutrient utilization is essential for sustainable aquaculture, as feed represents the major production cost and poor metabolic efficiency leads to environmental nutrient discharge. Conventional evaluation methods, including growth performance and feed conversion ratio, provide limited and low-frequency data, failing to capture rapid metabolic changes in individual fish. Implantable biosensors offer a promising solution by enabling continuous, in vivo monitoring of key metabolites such as glucose, an important indicator of energy balance and stress. This article outlines the principles of electrochemical and optical implantable biosensors and their application in real-time glucose monitoring in fish. These systems link feeding regimes and environmental conditions with physiological responses, supporting precision feeding and welfare assessment. Although challenges such as long-term stability and biocompatibility remain, advances in sensor technology and data analytics highlight the strong potential of implantable biosensors to enhance efficiency, sustainability, and productivity in modern aquaculture.