High or low current in a high-tech tank?
Is high or low current better in a planted tank or does it matter? I have seen responses both ways. Low current to mimic slow-flowing river tributaries some fish are native to and to reduce CO2 loss. High current to reduce algae and improve CO2 and nutrient distribution.
Does the current matter if there is good circulation? Why would higher current help prevent BBA, as mentioned in some posts?
55 planted, medium light, EI dosing, pressurized CO2.
it all depends on the plants you keep but most species of plants prefer low current.
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.
Very interesting numbers Byron, thank you. And thanks to all others for your responses as well!
Definitely different opinions on TFK than on another forum that focuses on plants! I never understood the logic about "more flow (and circulation) is necessary in a high-tech tank". The high-flow school of thought never made sense to me, always seeming counter-intuitive. I can see some slight current/circulation helpful to help move dissolved CO2 and nutrients around, but nothing near the velocities some people advocate.
Just noticed I missed the algae and high current point. I have also read that increasing the water movement is one means of combating brush algae. I've not done this, and have no intention of doing so, as the current would annoy my fish (and plants if one accepts that view), but I have to question it; I have brush algae on the filter tubes in the tanks with canister filters, where water movement is fastest. This would seem to contradict.:-)
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