Ideally every leaf on every plant will be moving a little from the water current. That will keep the plant supplied as well as possible with CO2 and other nutrients, and help keep the leaves clean. The Koralia type powerheads are very good for water circulation because they don't create such a hard blast of water, but they move a lot more water at a slower speed.
Too much water movement would be that which interferes with the growth of the plant, or uproots newly set out plants or forces the plants to grow in a direction away from the flow direction.
Last edited by Hoppy; 04-13-2013 at 06:29 PM.