I did a poll a while back to find out what most fish keepers like as far as turn over rates and the results were
**turn overs per hour**
4-6x with sump filtration
6-8x with canister filtration
8-10+ with HOB filtration
In my personal 125 I have a 1330gph pump running and i wish i had more.
But from the poll with your 120 tank and a sump you need a 480gph - 720gph after head height. So add around 200gph to those and that will get you close.