01-18-2012, 08:04 PM
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Circulation and current are two different aspects of water movement. Fish have differing needs with respect to current, and I believe that should always be foremost in deciding water movement. Won't say more on that since you've specifically asked in respect of planted tanks.
Circulation is important with respect to plants. Water movement removes particulate matter that settles on the leaves, it distributes nutrients to the leaves and roots, and a flow through the substrate is essential. It also ensures a more even temperature throughout the tank, though I suspect this is really only an issue when there is an extreme outside the tank.
One can have a healthy planted tank with no filter, thus relying solely on thermal currents and fish swimming to achieve the circulation. This is generally sufficient for the plants; if not, such a tank would not work, and they clearly do. Which brings us to consider if there is some point at which more current is detrimental, and this is where opinions differ. My own is that water flow should be suited to the fish, but no more, and minimal for this purpose is best.
Hiscock [Encyclopedia of Aquarium Plants, p. 116] writes that "mountain streams...are inhospitable places for aquatic plants. Fast-moving water constantly batters the leaves and removes useful nutrients, while high oxygen levels make life hard for many plants. ... CO2 systems would be ineffective in this type of aquarium, as the increased air/water exchange would remove much of the CO2 as soon as it was introduced." High oxygen levels make it difficult for plants to assimilate nutrients.
Walstad [Ecology of the Planted Aquarium] advocates much the same, supporting this view with scientific studies. CO2 diffuses very slowly in water, about 10,000 times slower than in air. And it takes aquarium plants four times longer to assimilate CO2 submersed than it does in the air (the aerial advantage). So the more rapid the water current, the less carbon will be taken up by the plants, and at the same time the more carbon will be lost through the gaseous exchange increased by spraybars, airstones and wet/dry filters. In one experiment, water movement of 1cm per second stimulated photosynthesis in Callitriche stagnalis, but faster water movement at 4 cm/second decreased photosynthesis by 13-29%. This is fairly significant.
Last edited by Byron; 01-18-2012 at 08:08 PM..