starting the 125 gallon setup

[Jaws]

You may want to reconsider such a large pump for your return too. I had a Mag 36 on my tank at one point and I wasn't happy with it. The slower the water is as it travels through the sump, the better chance your filtration equipment has at polishing the water, such as your skimmer, etc. If you can go with another closed loop or add a Tunze or two like the other guys recommended you might be a little better off.