<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by ninob:
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
Head height is always stated because most of these pumps are not used in closed circuits, rather in open sumps and refugiums where they have no back pressure at all to help them get the water back up to the tank. In a closed loop, overall pressure is the only concern as anonapersona pointed out.
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>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). <HR></BLOCKQUOTE>
No it would just stay still as both would cancel eachother out. If head pressure was indeed an issue in closed loops, when you turn the pump off the water would automatically flow backwards. The pump has to obviously work against any obstruction or restriction such as filter media, bends and accessories. That's also why some quality canisters have a larger intake tube than output to avoid negative pressure inside the canister, specially if the pump is situated after the media.