Avoiding resonance is crucial in design to save the system from high vibration levels that can be induced at the resonance frequency. When the excitation frequency equals one of the systems' natural frequencies resonance can happen. Therefore, to avoid resonance it is essential to evaluate the systems' natural frequencies and avoid exciting the system by one of them. However, controlling the excitation frequency is sometimes not feasible. So, in this case, shifting the natural frequency is useful to save the structure. The current article discusses the effect of adding or removing mass on the system's natural frequency of a water tower. Changing mass can be implemented by pumping or draining an amount of water. Modelling the controlled tower system is presented and studied. A Single Degree of Freedom SDOF model is used for examining the relation between the amount of pumped or drained water and the natural frequency shift. Different cases of frequency shift are studied. Dynamic response signals and Frequency Response Functions are obtained to demonstrate the impact on the system's natural frequency. The control system layout is presented. The article concludes with results that can be useful practically in saving the tower structure. The natural frequency can be shifted significantly by controlling the amount of water.