What's the max head height of the
pump (the max height the pump can pump the water to, and GPH at that height)? The GPH rating on the front of the box is going to be what the pump can do at a 0-1 foot head height, it drops off from there. How high is the water going to need to be pumped up from the reservoir level, to the highest point of the water level in the system?
P.S. Keep on mind that some system designs will have more back pressure than is intended by the pump ratings/spec's.