Exactly. There should be a distinction between filtration and water turnover (...mmm, turnovers...) which is directly related to circulation or water movement in other words. I thiink most of our filters accomplish this goal, but in larger tanks, this may be more of an issue. I, for instance, use a powerhead in my 90 gallon to provide additional circulation. I've noticed my plants grow better if they have good circulation, and areas with less circulation tend to accumulate algae faster. The plants gently swaying in the flow also provide movement which adds to the aquascape, I think.