Originally Posted by Diana
There is a balance between too much and not enough. As noted above, the tank does need water movement. At the surface, water movement enhances gas exchange, which can ADD CO2 in the right circumstances and reduce surface scum.
Deeper in the tank water movement keeps the fertilizers and so on in motion, bringing them closer to the plants, and keeps the debris in motion, bringing it closer to the filter intake for removal.
If you are adding CO2, pressurized or DIY, then too much water movement can release some of the CO2 to the air. But even then the tank needs some water movement. Adding CO2 is not adding oxygen for the livestock. There still needs to be an exchange.
How much is 'not enough' or 'too much' will depend on a lot of things, it is best to watch the fish and plants while you try different levels, and different arrangements of the equipment.
Fish from a fast flowing stream will thrive in fast moving water, fish from a lake, or slower moving stream will not like that much water movement. Fish can move to different areas of the tank. If they are always hanging out where the water movement is the strongest, I would add more water movement. If they are always hiding in the slowest moving corners then I would reduce the water movement. Fish that spend time in both areas are telling you the tank is set up just right for them.
I am looking at my Congo Tetras, and one is hanging out really close to the power head, I think he is going to get out his surfboard! The others are mid tank, in some water movement, just not right in front of the PH. The Loaches occasionally 'surf' the PH, but mostly play around on the bottom.
Too much water movement stresses the plants, doing a small amount of tissue damage. Gently waving plants seems to be a good amount of water movement, but, like fish, it depends on the plant. I have some Pennywort that is going crazy right on top of a power head, water is rippling as much as 1/2" up and down. Bolbitis seems to thrive right in the path of fast moving water.