When I used a Fluval 304 on a 29 gallon tank I found that the flow it produced would gradually decrease as the filter got dirty. So after a couple of weeks I would get a lot less flow than I started with. I had other problems with that filter too, primarily problems priming it, so I got rid of it. Now I use a Rena Filstar XP3 on my now 45 gallon tank, and it works much better, giving a lot more flow rate and the rate doesn't drop of nearly as fast.
I use a Koralia Model 1, which is placed in the back right corner, near the bottom, where it blows along the rear glass. This gives me good circular flow around the tank. The filter output is near the top of the tank, in the same corner as the Koralia, but the filter output is directed slightly upwards, so it ripples the surface. The filter input is at the bottom of the tank, also in the same corner. This results in giving me some top to bottom current to supplement the circular current. My CO2 goes into a very small powerhead located so it blows its output directly into the "cage" around the Koralia rotor. The powerhead rotor has holes melted in it to give a "needlewheel" effect. The result of all of this is that my tank is usually full of very small CO2 bubbles, and the drop checker indicates green pretty fast in the morning, probably as fast as it can react to a change in CO2 concentration.
I don't claim this is the best way to get good circulation and CO2 distribution, but it seems to work well. I should also mention that my aquascape consistes of a big wood "sculpture" in the middle, with plants gathered around it, and the surrounding substrate covered with low growing crypts. Thus, no plant mass to interfere with the circulation.
This may sound good, but I am still struggling with BBA problems.