In this talk, we demonstrate how to derive temporal decay rates of weak solutions (also mild solutions but not much) to dissipative equations such as the incompressible Navier-Stokes equations, SQG, and MHDs. The main tool is the Fourier splitting method with initial data in spaces of negative regularity. This is joint work with Jinwook Jung, Kyungkeun Kang, and Jaeyong Shin.