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Like everything in this hobby- it depends...

If you have too little water movement, adding a little more, especially directed in the right way, can be very beneficial.

If the fish are already getting out their surfboards, then you do not need more water movement.

Signs of too little water movement:
~Tie several pieces of thread to a stick that is long enough to reach the bottom of the tank. (a chopstick for smaller tanks, for example). Hold this in the tank in many locations and watch the thread. Does it flutter? Good. Does it wave madly? Bad (except right out of the filter). Does it not move, or barely move? Bad. This can also help you figure out where to locate and aim a circulation pump.
~Surface scum. Easiest to see if it is sort of oily, might look like a rainbow. Bits of 'stuff' on the surface that does not move around enough to get down to the filter intake.
~Fish piping (hanging at the surface and gulping air)
~Plants not moving or barely moving.
~Debris pretty much falling all over, not much is making it into the filter intake.
~Flat sided fish like Angels and Gouramis do fine in this sort of set up. (Especially Anabantoids, who can breath at the surface)
~Floating plants may be almost anywhere on the surface, though usually not right at the filter outlet.
~Generally too little water movement is common in tanks with less than 5 times the tank volume of circulation per hour, or poorly located filter inlet and outlet so the water does not circulate well around the tank.

Signs of too much water movement:
~More than gentle ripples at the surface.
~Fish hiding in calm areas, not swimming around much or having a hard time swimming. This is especially noticeable with large flat sided fish like Angels and Gouramis.
~Plants moving a lot, or always bent in one direction, leaning. Too much water movement can damage the leaves, too.
~Debris is mostly blown into a few stagnant corners, or taken in by the filter. Mostly the floor will be pretty clean. (this is good)
~Duckweed does not like your tank. A few bits may cling to driftwood or other things above the surface.
~Hillstream Loaches like this sort of set up, but it does call for small, sturdy plants.
~Generally Hillstream Loach tanks are fine with 20x, but most average tanks are getting too much water movement when filters and power heads are over 10x.

Just right:
~Fish that come from a very calm back water will not like quite as much water movement as fish that come from average or faster moving water, but ought to find enough places that they are OK. Fish from faster moving water might not find enough oxygen in this sort of set up.
~Plants gently sway.
~Debris mostly gets moved to the filter intake, but there might be a few spots you need to target when you clean the tank.
~Surface gently ripples, enough movement that there is no surface scum. Usually floating plants will get pushed into a corner or to the sides.
~I generally aim at 10x based on the manufacturer's ratings of their filters and power heads. (Yes, I hear all the laughter). I know I am not getting all the flow they say their equipment puts out, but at least it is a start.

Other notes:
When adding to the circulation of a tank, look for the slowest moving water and see if you can get it going in the same direction as the rest of the water. Do not try to create turbulence, or crossing currants. Try to enhance existing flow.
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