If you are using CO2, and an external reactor, so the incoming water has the CO2 dissolved in it, I would put the spray bar at the bottom of the tank, pointed towards the front. That puts the CO2 down where it can rise up through the plants, giving them the best shot at getting some of it before it leaves the tank at the water surface. If you make the spray holes in the spray bar have a combined area significantly less than the cross section area of the spray bar tube, every hole should get about the same flow through it. And, the larger those holes are, the slower the flow will be coming out of the holes. But, if the flow is too slow, the water and CO2 won't reach all the way across the tank.