Hello,
I believe the 1.25" hole and the Mag 9 are enough, at least to get started and for some time. The higher the flow rate the better but you need to make sure that the flow into the display tank is also evenly distributed and not focused heavily on one single area. You can do this with multiple outlets. A good example can be seen
here.
Do you have a good overflow to collect water over a large surface area? How is your design like? Tell us a bit more about your setup. Like do you use a sump and what are the dimensions and other equipment you use.
Titus