Sorry, but I don't agree with Northstar....
The pump doesn't have to match the overflow rate but if you think about it all the overflows are, is overflows...You don't want a pump that pushes too close to the overflow rate because any fouling or blockage of the overflow will make the tank overflow...the overflows will only drain the amount of water your pump is pushing up to the tank....the flow into the tank will all go down the drain with the level controlled by the overflow wall...The danger is when you start messing with the overflow drains by throttling down on them...then when just the right little hunk of algae, wasted food or snail will slow it just that critical amount more and you will be explaining the flooded living room to the wife...