Many aquarists keeping freshwater planted tanks add $\ce{CO2}$ from a pressurized canister to their aquaria to promote plant growth. Injection is at a slow rate to get dissolved $\ce{CO2}$ in the neighborhood of 30 ppm. Methods vary, but a ceramic discs to release a very fine mist of bubbles are popular, as are $\ce{CO2}$ reactors which trap the bubbles in a bell or canister and stir them around with a water pump.

In any case, within the hobby there is an odd fixation on getting the gas bubbles to dissolve completely, I suppose on the reasoning that a bubble that pops at the surface is $\ce{CO2}$ that escaped to air rather than being dissolved in the water.

I'm skeptical of this logic -- I'd think while the bubble is dissolving, water vapor and whatever other dissolved gasses will be diffusing into the bubble. At some point the bubble becomes just humid air in equilibrium with the surrounding water, and no amount of stirring will get more $\ce{CO2}$ out of it. This is just my guess, based on casual observations where the bubbles will diminish to less than 10% of their initial volume within 30 seconds, and thereafter they don't seem to get any smaller.


Is that correct, and does it mean the aquarist's quest for a "100% efficient" $\ce{CO2}$ bubbler that never lets bubbles pop at the surface is doomed to eventual failure?

And if so, what are the basic principles that describe this equilibrium and how can we figure how much $\ce{CO2}$ can be extracted from a bubble, and how big the bubble will be after it's become just humid air?

If not, why does it seem the bubbles will rapidly dissolve in the water at first, but never fully dissolve even though the solution is far from saturated?

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    $\begingroup$ There is a limit as to how much $\ce{CO2}$ will dissolve into water. I'd wonder about aggressively adding $\ce{CO2}$ in an aquarium with fish or other oxygen dependent animals. The excess $\ce{CO2}$ will sparge dissolved oxygen from the water. $\endgroup$
    – MaxW
    Dec 22 '20 at 22:35
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    $\begingroup$ @MaxW The rule of thumb in the hobby is to aim for 30 ppm, though it can vary by species. The $\ce{O2}$ released from photosynthesis is impressive, so there's plenty of oxygen if the tank is running well. I'm no biologist but I always figured excess $\ce{CO2}$ will kill the livestock directly by acidosis before oxygen deprivation does. $\endgroup$
    – Phil Frost
    Dec 23 '20 at 1:09
  • $\begingroup$ @Mithoron Thanks, I'm here to learn! Perhaps you could put that in an answer? $\endgroup$
    – Phil Frost
    Dec 23 '20 at 1:29
  • $\begingroup$ Maybe, if post won't get closed and no one answers it anytime soon. It's just that a bubble with other gasses could also dissolve, unless they are too close to saturation point - so you may be not wrong after all if that's the case. $\endgroup$
    – Mithoron
    Dec 23 '20 at 1:36
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    $\begingroup$ Questions: (1) Fresh water or salt water? (2) Are there fish or other animals that depend on dissolved oxygen in the aquarium? (3) Is the pH adjusted regularily in the aquarium? (4) To what value is pH adjusted? $\endgroup$
    – MaxW
    Dec 23 '20 at 10:40

To improve your CO2 intake and not lower dissolved oxygen levels, an apparent solution is to make the water in your fish tank more alkaline and also consider increasing the surface turbulence.

The logic is that there is an equilibrium based dissociation of CO2 in water creating Carbonic acid. However, in the presence of OH- ions, the formation of an insoluble carbonate (like CaCO3) will continue to drive the reaction to the right.

On the effect of turbulence from an air pump agitation of the water or perhaps a waterfall creating mist, will capture the carbon dioxide.

So, as solutions consider adding alkaline stones. There is also a supporting article: Limestone Rocks in Aquariums and Water pH, to quote:

Have you set up an aquarium and then noticed that the pH of the water continues to rise? Could rocks or gravel that were added as decor in the aquarium affect the water pH? Yes. If your rocks are actually limestone, they are the cause for the pH elevation in your aquarium water. Limestone is calcareous (contains calcium) and is known for its ability to both harden the water and increase the pH. In fact, if someone asks how to raise water pH, one method we suggest is to place crushed limestone, coral, oyster shell or any highly calcareous material in the filter.

On the waterfall idea a reference, to quote:

Speaking of pond health and waterfalls, they're a great way to aerate your pond, but you might not want to overestimate how much oxygen is getting into your pond from the waterfall. Specifically, the larger your pond is, the higher the waterfall would need to be to effectively aerate it alone.

Hope this helps.

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    $\begingroup$ This is interesting, but it doesn't really answer the question, "Will CO2 bubbles ever fully dissolve in water?" $\endgroup$
    – Phil Frost
    Dec 23 '20 at 15:36
  • $\begingroup$ On a practical note, increasing the pH and the hardness of the water interferes with nutrient uptake for the tropical species commonly used in freshwater planted aquaria which are adapted to soft, acidic water; and I don't think carbonate is bioavailable for photosynthesis. Oxygen saturation is really not a concern: photosynthesis converts $\ce{CO2}$ and water into carbohydrates and $\ce{O2}$, which is so abundant it bubbles off the plants. It's not difficult to get oxygen saturation over 100%: the concern for livestock is the toxicity of $\ce{CO2}$ but 30 ppm isn't harmful for most species. $\endgroup$
    – Phil Frost
    Dec 23 '20 at 16:24
  • $\begingroup$ Phil getting CO2 gas to dissolve becomes increasingly difficult when it has been effectively 'salted out' (because of other dissolved compounds) or due to elevated water temperatures. However, as the chemistry discussed in my answer notes, no matter how reduced this level, once dissolved in an alkaline solution, it is removed (via, for example, CaCO3) and more CO2 can be dissolved. The release of acidic products from organisms in the fish tank can release this stored CO2. $\endgroup$
    – AJKOER
    Dec 23 '20 at 16:44
  • $\begingroup$ That makes sense, but my question isn't about the total amount of CO2 in the water, but rather the fate of an individual bubble. My observation is that if I take a fresh tank, which could absorb many, many grams of CO2, and trap just a single CO2 bubble under a bell in that tank, the bubble will be mostly dissolved in about 30 seconds and then get no smaller. Surely this isn't because the solution is saturated, because adding a second bubble the same thing happens: rapidly mostly dissolved, then no further visible process. Why is that? $\endgroup$
    – Phil Frost
    Dec 23 '20 at 16:55
  • $\begingroup$ In the frame of the question posed by OP, this is waste of CO2. $\endgroup$
    – Alchimista
    Dec 24 '20 at 9:08

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