When you say 20 feet are you talking about the lenght the water must travel, or the total height? The higher you are pushing the water, the more powerful the pump must be. Some pumps are designed for high height (or head), others are designed for low head and are not very efficient pumping water high heights. The horizontal distance your water travels is much less a factor than the vertical distance when sizing a pump. I prefer internal pumps as they are easy to plumb. I like mag-drives (by Danner) for smaller jobs (up to 1500 gph or so). Danner now makes Hy-Drive pumps for larger jobs (3200 gph and up). Sequence makes a variety of good pumps too, some designed for low head, some for high.
How much water to do you want to move per hour, and how high will the water be going from the pump to the tank?
Pat
Amazon Stingrays