The current study was designed to determine the effect of magnetic water technology and different stocking densities in a recirculating aquaculture system (RAS) on water quality parameters and the growth performance of the Nile tilapia (Oreochromis niloticus). RAS mainly consists of rectangular tanks for fish rearing, two centrifugal pumps (power  = 0.5hp, Hmax = 35m, Qmax = 2m³/ h) used for recirculating the water in RAS, a mechanical filter, a biofilter, an aeration system, pipes, valves, and other fittings. Magnetic water with a degree of 14,500 gauss (1.45 Tesla) was used in comparison with normal water (0 gauss). The Nile tilapia fingerlings with an average initial weight of 5.4± 0.31g were randomly distributed. The experiment was based on a 3×2×3 factorial design with two stocking densities (50  and 100 fingerlings/ 0.5m³), and each group was duplicated and reared in three types of water: control water (CW), magnetic water (MW), and non-magnetic water (NMW). The experimental fish were fed a commercial diet (30% protein) at a feeding rate of 4% of body weight for all groups. Water quality parameters, growth performance parameters, feed use, and chemical composition were calculated in all groups at the end of the experiment after six weeks. Results indicated that using a recirculating aquaculture system (RAS) with natural bacteria as a biological filter is highly effective in improving the water quality characteristics, especially eliminating ammonia. However, the use of magnetization technology to treat fresh water culture does not make a significant difference in improving the water quality. Furthermore, the water recycling system not only saves water throughout the production period but also improves the growth performance of the Nile tilapia fingerlings.