This is from Randy's article.
In my experience with CO2, aerating the water will only help raise the ph only so much and mabey not enough if the air surounding the tank contains high levels of CO2. I pull air into a 10 gal. 24/7 and if I don't crack the window of the room the DT is in, the ph will stay down near 7.8 at the end of the day. Below 7.6 in the am. Keep in mind that I am in a small ranch and the tank is in my bedroom with the door closed at night. 3200 ppm CO2 in the am. FOWLR, so I am not as concerned as most with corals. Also I am using a Aquaclear filter which dumps water into the tank. I don't know if this is dragging the gas into the water with the pouring of water from the filter, causing the CO2 levels to be higher than if the filter had a closed loop. My ph swings can be anywhere between 7.6 and 8.1 depending how long I keep the window open for. Example is that I left the window open yesterday from early morning until 9 pm and my ph climbed from 7.65 to 8.1. This morning at 6 am it was 7.78. The Clown, Damsel, Hermitt, and Pep. Shrimp do not seem to be effected by these swings. I will be starting a 75 gal. FOWLR soon and will do more testing with a closed loop, skimmer, and levels of CO2 to go by.
Carbon Dioxide and pH
The pH of marine aquarium water is intimately tied to the amount of carbon dioxide dissolved in the water. It is also tied to the alkalinity. In fact, if water is fully aerated (that is, it is in full equilibrium with normal air) then the pH is exactly determined by the carbonate alkalinity The higher the alkalinity, the higher the pH. Figure 1 shows this relationship for seawater equilibrated with normal air (350 ppm carbon dioxide), and equilibrated with air having extra carbon dioxide as might be present in a home (1000 ppm). Clearly, the pH is lower at any given alkalinity when the carbon dioxide is raised. It is this excess carbon dioxide that leads to most low pH problems for reef aquarists.
A simple way to think of this relationship is as follows. Carbon dioxide in the air is present as CO2. When it dissolves into water, it becomes carbonic acid, H2CO3:
3. CO2 + H2O àH2CO3
The amount of H2CO3 in the water (when fully aerated) is not dependent on pH, but only on the amount of carbon dioxide in the air (and somewhat on other factors, such as temperature and salinity). For systems not at equilibrium with the air around them, which includes many reef aquaria, the aquarium can be thought of "as if" it were in equilibrium with a certain amount of CO2 in the air, which is effectively defined by the amount of H2CO3 in the water. Consequently, if an aquarium (or the air it is being equilibrated with) has "excess CO2" in it, that means that it has excess H2CO3. This excess H2CO3, in turn, means the pH will fall, as shown below.
Seawater contains a mixture of carbonic acid, bicarbonate, and carbonate that are always in equilibrium with each other:
4. H2CO3 ßàH+ + HCO3- ßà2H+ + CO3--
Equation 4 shows that if an aquarium has excess H2CO3, some if it dissociates (breaks apart) into more H+, HCO3-, and CO3--. Consequently, because of this extra H+, the pH will be lower than if there were less CO2/H2CO3 in it. If seawater has a huge excess of CO2, the pH can be as low as pH 4-6. Equilibrating my aquarium water with carbon dioxide at 1 atmosphere resulted in a pH of 5.0, although that low a value would be unlikely to be attained in a reef aquarium as the substrate and coral skeletons would buffer it as they dissolved. My aquarium water in equilibrium with 1 atmosphere of carbon dioxide and excess solid aragonite (a crystalline form of calcium carbonate that is the same form present in coral skeletons) resulted in a pH of 5.8.
Figures 2-5 show graphically some of the ways of raising pH in aquaria. For example, if the aquarium has an alkalinity of 3 meq/L (8.4 dKH) and has a pH of 7.93, then the aquarium must have excess CO2 in it (or else the pH would be just over 8.3). Ways to raise pH include:
Aerating the water with "normal air," driving out the excess carbon dioxide, will move the aquarium parameters along the green line of Figure 3, raising pH to just over pH 8.3. This effect is also what would happen if the growth of macroalgae were used to absorb some of the excess carbon dioxide, although it is rare for that effect to be able to move it all the way along the green line to above pH 8.3.
Raising the alkalinity, even if it still has the "excess CO2" in it, will raise pH by moving the aquarium parameters along the green line in Figure 4, to a pH of about 8.1 at an alkalinity of 4.5 meq/L (12.6 dKH).
Using limewater (kalkwasser) to deplete the excess CO2 (to normal levels), and also to raise the alkalinity (to 4 meq/L) could move the curve along the green line in Figure 5, resulting in a pH over 8.4 and an alkalinity of 4 meq/L (11.2 dKH).
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