I haven't bought a pump yet, and there is no water in the tank. The waters surface will be at most 55 inches from the top of the pump (this assumes the pump is flush with the bottom of the tank and the water fills the tank to the brim). More realistically, it will be 52 inches less the height of the pump. I plan on purchasing a pump based on the flow rate I am looking for. I haven't read up enough yet, but I'm thinking 5 times volume is 600 GPH.