Hello,
I don't think your Mag 9 would overflow your tank. But always put a ball valve at the pump's outlet so you can have control of the flow. Best yet put a tru-union there also so you can just shut off the valve, remove the pump for servicing if required. Your 1-1/2" overflow size should handle the return flow from the Mag 9.
If the run from the pump to the display tank isn't too long, you can use the same diameter as the pump's outlet.
Titus
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