If that indicates a single 1" bulkhead, a Mag 18 is WAY too much flow for the tank. 1" bulkhead should have a theoretical max of 1000 gph, and most will only flow about 800 gph due to various plumbing restrictions. A mag 7 or 9.5 would be better suited for your application. IF you have to throttle back the pump, (say the 9.5) then this pic from Melev's site shows the best way to do it:
More wonderful sump info can be found here:
http://www.melevsreef.com/what_sump.html