Nitrate buildup in recirculating aquaculture systems (RAS) poses risks to fish health and water quality, threatening environmental sustainability. This study explores agricultural by-products as cost-effective carbon sources for heterotrophic denitrification, offering a sustainable alternative to traditional nitrate management methods. By evaluating carbon release dynamics and material composition, this research provides insights for optimizing nitrate removal processes in RAS, promoting more effective and sustainable aquaculture practices. Four by-products—water weed, moringa husks, almond hulls, and wine corks—were processed and tested for carbon release over 120 hours in distilled water. Chemical oxygen demand (COD) was measured, and compositional analyses of cellulose, hemicellulose, and lignin were conducted. Carbon release kinetics were modeled using second-order and Ritger-Peppas equations. Almond hulls exhibited the highest carbon release, followed by water weed, with moringa husks and wine corks showing significantly lower release rates. The Ritger-Peppas model indicated diffusion as the primary release mechanism, contributing to steady carbon availability essential for stable denitrification. The kinetic models provide insights, helping to optimize carbon dosing in denitrification applications.