Reduction in loss of head/pressure caused by friction of the tube walls...
Someone posted a chart or some figures of headloss for 1/2 tubing on a Ehiem1250 IIRC and the results were frightening!. A few feet of tube can realy knock your head off!
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PS, it was a thread about 'best pumps' or 'pump p~curves' I think. I'll post a link if I can find it...