a mag 3 would do 350 gph with no head pressure. In the real world you have to pump the water up to the tank and that lift creates resistance. Also the piping itself between the pump and the outlet creates resistance. This resistance decreases the capacity that the pump can push. So, even though the mag 7 is rated to push 700 gph at 0 head it will probably be pushing really close to the 400 gph that your overflow is rated for once you get it in place with all the plumbing, ect. HTH