Choose one with a flow rate of 3-10x display tank size after head losses..
Example..
You have a 90G tank (90 x 5 = 450GPH, 90 x 10 = 900GPH) and the vertical distance from the sump to the top of the display tank is 5 feet..
So pick a pump that can flow roughly 450 to 900 GPH at 5ft of head..
All pumps will have a chart showing their flow rates at different head heights..
Then if you plan on running a return manifold with carbon/GFO/biopellet reactors,etc.. of it you need to add some extra for those..
Then cost factors.. controllable pumps,etc... are all other decisions you can make..
controllable is nice because you can well.. adjust the flow as you see fit..
blah blah blah...