Not sure if the question was geared towards a filter or just flow within the tank, but I'll add my 2 cents for both all together.
Regarding the filter performance bigger (within reason) is always better, but not necessity. IMO the important thing is for you to ensure the bio capacity can handle the livestock load. Once this is matched up, then you can supplement flow with additional powerheads.
How I gauge flow in my tank is not based on a set number, but how overall plant movement is within the tank. Everything should have a nice gentle sway or better yet if you were to disturb the substrate dust should not fall back to the same spot if that makes sense. It should want to move in the direction of the intake of you filter inflow. It might not make it in one go, but this is the general concept I use for my tanks with regards to flow.
The 穀精 Junkie
Last edited by mrkookm; 02-05-2013 at 02:36 PM.