QUOTE=houseofcards;407432]I really don't think you need flow in the sense that you see the water actually moving that much. In most of our tanks that are 2 to 4 feet big, how much flow do you need to move dissolved ferts and co2 around. If your talking about algae control, I haven't seen any proof that algae grows more in areas with less flow. I've seen algae on people's tank right inside the co2 diffusor and right inside their return tubes. I have to believe there is flow there. Look at a pond outside. How much algae grows right on the rocks in a waterfall. There isn't one answer. It depends on your maintenance habits and other variables. Hobby why does Barr recommend alot of flow? By the way in terms of biological filtration this is a direct quote from Eheim:
Hi Houseofcards, I have one comment to make regarding why you see algae forming on the inside walls of plumbing in filter tubing that that is a phenomenon called "wall drag" or "fluid slip". Where as in a pipe there are two flow rates actually occurring. One is in the centre of the pipe which is what we all see and preceive as the only flow. The other is caused by the "wall drag" this link my describe it better than I can;
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=14761
essentially what happens is on the inside of the pipes or tubes there are microscopic grooves where water traveling on the outside of the diameter of the water column get trapped in these grooves. The water that is on the inside track simply just slides over top the trapped water. Thus creating a wonderful little home for algae grow.
I personally believe that flow is required in an aquarium no matter what the nature of it (fish only, planted, or reef). The same basic principals or envolved, and that is we want to remove debris and toxic wastes from the inhabitants.
On the other hand too much direct flow could work against the aquarium environment where the plants cannot "catch" their food from the water column. So that leaves us with a nice gentle dispersed flow throughout the tank via a spray bar or multiple water return lines in order to get the job done. That's not too say that it is an absolute must have, after all if it works for some to have very little flow than that's all that really matters.
I hope this as shed a little light onto the subject.
Cheers to all!