For the common return pumps (Eheim, Danner, Iwaki, etc.) I see "head ratings". It's my understanding that this rating refers to the height to which a pump can push water. What if I want to pull water uphill, though? If a return pump has a head rating of 5 ft, could I have the pump at the height where I want to water to exit, and pull it up from 3 or 4 (or a theoretical maximum of 5) feet below?