This work deals with the thermal stratification problem inside a vertical storage tank used in a Solar Water Heating System (SWHS). The study draws on established Unsteady 3-Dimensional Computational Fluid Dynamics (3D-CFD) tools, and results are validated with published experimental work. However, some changes on tank top-bottom shapes are made, and the stratification behavior is accordingly examined. Although these changes may seem a bit minimal, they are very interesting since they have a quite noticeable impact on the storage tank efficiency, and the most performing designs could be consequently selected. The 3D-CFD results of all proposed designs are compared through some mixture and stratification evaluation criteria, such as the outlet temperature. The general trend is that a spherical top-bottom shape produces a superior thermal performance.