shagooo:
A reservoir tank is analogous to an electrical capacitor; it's meant as a temporary storage of fluid for a specific purpose. If flow fluctuation is your concern, then enough residence time is built in to smooth out the flow. If there are lapses of flow during operation, then those time periods are taken care of by building in enough capacitance capacity in the reservoir to take up the slack in fluid flow.
So it all depends on what you are trying to do or the nature of your fluid application. Normally a centrifugal pump is used in a steady-state process, so there is no need for a reservoir tank - either before the pump or after the pump.
Perhaps if you explain your reasons for trying to apply a reservoir, we may get a better handle on the problem - if there is one.