While I may disagree with joe on the first part of this, there is some truth to it. If you are using an overflow box rated at 1500gph, and your pump is only 600gph, pending on the overflow box design, it should work as long as your box doesn't have problems staying primed/syphon in tact. Keep in mind that your overflow box will only pull as much overflow in as the return pump keeps in the display tank. That being said, the 'ideal' situation is for both of them to be rated the same (overflow = 1500gph / pump = 1500gph). Also keep in mind that you want VERY LOW flow in your fuge, just enough to keep the surface agitated (1-2/hr)...