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Mag 18 is too powerful for the return from the sump. You should look at about 5-6x turnover from the sump. the Mag pumps will also add heat to the water. You would be better off getting an external pump for a return and setting up a seperate Closed loop. Is the tank Reef Ready or will you build standpipes for it?
We ran a Mag15 (Mag 12 with an 18 impeller) on our 120. By the time you have head loss that brought the tank to just about exactly 1200gph (which is what the standard sized overflows accomodate). This was when the sump was under the stand, so it was a fairly normal split configuration. My 120 was drilled with 2 holes for 3/4" returns and 2 holes for 1" drains (each hole is up-sized to accomodate the requisite bulkhead fitting).
The Mag pumps do introduce a fair amount of heat into the water (whether submerged or external) because they're water cooled. What you'd want to use, in order to eliminate that heat transfer, is a pump that can only be run externally because those are generally air cooled instead.