pump selection

Quick question, (I hope)

I have a 300gph powerhead, made by Aquaclear I think. It really moves a
large volume of water if I just drop it in the water. I decided to
build a canister filter underneath my tank. I bought some 1/2" flexable
clear tubing to move the water. By just adding 4 foot of tubing to the
pump, flow drops significantly. It won't even pump water from a tub on
the floor into the top of my tank. If I restrict the flow at all, the
pump chokes. Much to my suprise, this pump ain't gonna work. What
should I use for a 55g?

my aquarium page, info and pics at:
www.geocities.com/spiral_72/Spirals_page.html