This work uses a PID sliding surface with nonlinear tuning parameters to design a Sliding Mode Controller. However, linear sliding surfaces have certain disadvantages; for example, when the sliding surface is linear, the magnitude of the control input required to keep the system states on the sliding surface usually increases in direct proportion to the magnitude of the tracking error. Thus, the nonlinear sliding surface may perform better if the nonlinearity is appropriately selected. The proposed controllers will be applied to a nonlinear mixing tank to test its performance. It will be compared against a PID controller and an SMC (Sliding mode control) through the simulation and performance indices, ISE, TVu, maximum overshoot, and settling time.