Based on the responses, you need to clarify your question. I assume that you mean that you are concerned with the changes in the level of the liquid in the reservoir on the pump. Why does it matter? Have you determined that those changes will impact the discharge pressure/flow of the pump to an extent that is not acceptable? If so, pardal's idea would work. If you can't do that, flow or pressure regulation on the pump discharge - a Griswold flow regulator or a self contained pressure regulator would be one approach thats not too expensive in small systems.