I didn't like my Mag 18 for my closed loop... much more usefull in a submersed sump return situation but I don't think it's powerful enough for your intentions.
Here's a link that might help in choosing a pump:
http://reefs.org/library/pumps/
Concentrate more on in-tank flow... you just need to get your water too and from your sump even if that means only 4-500 gph.