Recirculating Aquaculture Systems (RAS) are advanced, land-based fish farming setups that continuously filter and reuse water in a closed loop, drastically cutting water use and allowing high-density farming with precise environmental control. These systems use mechanical filtration for solids, biofilters for ammonia, and disinfection (UV/ozone) to maintain optimal water quality, enabling sustainable fish production near consumers with minimal waste discharge, though they require high energy and initial investment.