Sorry but I have to disagree. They don't offset each other completely. If you buy an external water pump, many come with a chart and the gph will drop as they are placed further down. Look here : http://www.drsfostersmith.com/produc...fm?pCatId=4614
The intake/input is assisted by gravity (syphon effect) and the output works against gravity. If they cancel each other out totally, you don't need a pump. The water will just syphon down to the canister and go back up to the tank (assuming there is no media).