In this paper fractional order PID (FOPID) is developed for a three tank system. Fractional-order controllers are based on the fast developing scientific arena called fractional-order calculus. These concepts are valid in solving many scientific and engineering problems, including robust control system design. The fractional PID is a progress of the conventional PID controller and new tuning techniques are possible due to improved accuracy of the fractional-order models. The results of DS based FOPID controller compared with the Ziegler Nichols (ZN) closed loop tuning provides a remarkable improvement in tracking a given set point and disturbance rejection. The results of FOPID controller tuned with Differential Search(DS) optimization is also compared with the response of PID controller tuned by PSO and ACO optimization, which shows better control with improved performance measures of FOPID on the Three tank system. © Research India Publications.