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No, you can't have too much O2 in the tank. And it would be very difficult to oversaturate the water with oxygen the way a typical fish tank is set up.
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Referring solely to live plants, it is possible to have too much oxygen, and this can inhibit the uptake of certain nutrients by the plants. When dissolved oxygen levels are high, several nutrients, especially iron, bind with oxygen and become too large to be assimilated by the plants.
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What will remove CO2? If you're referring to aerating the tank, then yes and no. Aerating will remove CO2 IF CO2 is being dosed into the tank, OR if there's enough fish that the CO2 produced is higher than the equilibrium concentration. However, if neither of those are the case, then aerating will replenish the CO2 that's being used by the plants.
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This topic has been debated on plant forums. The long-held view is that water movement will drive CO2 out and bring in excess oxygen. While a few notable authors have suggested otherwise, there has so far, to the best of my knowledge, been no concrete evidence that aeration in a natural (low-tech) method planted tank actually brings in CO2 rather than driving it out faster.
The sediment water contains much higher concentrations of CO2 than the upper water, as much as 100 times more. Two main reasons account for this. First, the organics in the substrate will be broken down by various bacteria and as they decompose a lot of CO2 is produced. Second, the plants themselves cause CO2 in the substrate. Plants have large internal channels for the transportation of oxygen, and most of this oxygen is transported to the roots where it is released. [The oxygen released by the roots of floating plants diffuses directly into the water, and this is one of the major benefits of floating plants.] In the substrate, this oxygen combines with carbon and organic elements creating CO2 which enters the water is then taken up by the plant (through its leaves) in photosynthesis.
The uptake of CO2 from the water is a much slower process than from air. The diffusion of CO2 into water is 10,000 times slower than in air. This means that the CO2 molecules don't contact the plant leaves fast enough, limiting CO2 uptake. Plants with an aerial advantage, such as those floating or sending leaves above the water surface, take up far more CO2 and thus photosynthesize (grow) faster.
Water in equilibrium with air contains 0.5 mg/l of CO2, which is insufficient for most plants. In our natural method planted tanks, CO2 for aquarium plants is mostly derived from fish food and organic matter in the substrate. It is vital to limit loss of this natural CO2 which will occur through any measure that increases air-water mixing. The higher level of CO2 from the substrate compared to the air will be lost faster than the plants can take it up if there is increased surface disturbance.
Byron.
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