The study was conducted to evaluate the effects of magnetic water treatment technique on water parameters and the Nile tilapia (Oreochromis niloticus). Magnetic water at 2000 gausses was used compared with normal water for triplicate groups of fish, which reared with different stocking densities 15 fish / m3, 20 fish / m3 and 25 fish / m3 and each group was replicated with 5g / fry an average initial weight. Fish were fed on commercial diet (40% protein). The tanks in the two groups were supplied with air pumping and filtration system. The results revealed that dissolved oxygen, pH and total hardness values in magnetic water increased as compared to control water, but ammonia was inversely related to magnetic field, while total alkalinity and water temperature were not affected by treatment. The data indicated that growth performance, feed utilization, water parameters, some blood parameters and body composition improved and significantly in magnetic water treatment between different stocking densities. The results pointed that, the stocking density of 15 fish/m3 with magnetized water was the best system where it has high water quality, growth performance and blood parameters of O. niloticus compared with other stocking densities.