I just have one 6100 in the left front corner of a 4' tank pointing toward the center. Using the contoller it sends different levels of flow toward the middle and dispurses. Works great and the Tunze was a worthwhile investment. Made a big differnece.
The right rear corner has a Seio 1100 that's always on. I'm thinking of a second 6100 for that corner to change up the flow. Of course that involves upgrading the controller also so budgeting is a definite consideration.
I think it's more about changes in the flow moreso than amount of flow.