Most people reccomend 10 -20 times tank volume per gallon. So you should have 1400-2800 gph
. some of the flow can be accomplished with phs. The rest with the fluvals. Point the phs slightly up to ripple the surface. It helps keep DOCs from forming on the surface.
For gas exchange I would reccomend a sump/fuge. It increases the waters surface area, which aids in gas exchange. I also use mine to grow macro that helps with the nitrate cycle.