more circulation and she said having to much is bad.
I guess if you had something like 50 X the turnover of your tank that would be excessive but in no way is 400gph to much for a 40 gal
breeder. If this is the only thing you have in your tank moving the water I would say you have the minimum circulation amount at this point. You want good random flow. TO acheve this do what was mentioned above by having the powerheads shooting toward eachother or shooting toward a common point. The currents will mix and provide good random indirect movement vs a jet stream of flow like the water is right out of the PH.