A pump having an insert between an impeller and diffuser, where the insert includes an annulus that is in communication with fluid flow passages in the impeller and diffuser. The passages and annulus define a fluid flow path through the pump. Vanes are provided in the annulus that can pivot and vary the cross sectional area of the fluid flow path. Regulating the fluid flow path area alters the flow rate where the pump operates at its maximum efficiency. Thus by monitoring flow through the pump, the vanes can be adjusted so the flow rate of maximum efficiency corresponds to the actual flow rate.