bubbles, CO2 and O2 - The Planted Tank Forum
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post #1 of 21 (permalink) Old 09-12-2011, 05:12 PM Thread Starter
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Question bubbles, CO2 and O2

I belive that is generally accepted that adding bubbles to your aquaruim will help oxygenate the water. But I have never seen it mentioned whether it can add CO2 to your to your aquarium as well.

If you have a lowtech setup (as in not injecting CO2) with a low CO2 concentration, wouldn't adding bubbles add CO2 to the aquarium rather than 'gas it off' as it does in a system where CO2 is being injected?
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post #2 of 21 (permalink) Old 09-12-2011, 05:32 PM
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It will out gas the co2, if u have a small tank you have to use bubbles or powerhead to move the surface water, just turn up the co2. For larger tanks bubbles are not as important cause of the surface area, more surface area = more oxygen exchange. On my tank I have high co2 and don't use anything to airate the water, my tank is longer than it is tall and is 15 inches wide so I have alot of surface area.


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post #3 of 21 (permalink) Old 09-12-2011, 06:15 PM
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My means to aerate the water surface is from my marineland emperor 400 filter. It has enough water surface movement to create small ripples. I believe the 2 bio-wheels created enough O2 contact in the air. That's it. No air pump day or night. Using pressurized CO2 with 3.6 bps. Diffused the CO2 with a powerhead.
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post #4 of 21 (permalink) Old 09-12-2011, 06:22 PM
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SeaSlug, from what I've heard about CO2 it dissolves MUCH less readily in water than Oxygen does. So I think that the water movement that helps get oxygen in the water is much less effective at getting CO2 in the water. In a low tech situation I don't know if water movement would cause a net gain or net loss of CO2, but it doesn't seem like it would have a very strong effect either way. I don't know for sure, but that's my guess.

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post #5 of 21 (permalink) Old 09-12-2011, 06:35 PM
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Sorry no one here can listen to your questions...

It seems that you are asking if CO2 from the atmosphere could be absorbed if you injected "air" from a powerhead or pump into the aquarium... Since CO2 is readily available in the air... interesting question... CO2 is also in your tap water though... so changing water on a weekly basis with a lower light setup/low low fertilization or a good aquasoil should be sufficient for a low tech setup.

let me know if I was spot on with your question
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post #6 of 21 (permalink) Old 09-12-2011, 06:41 PM Thread Starter
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Quote:
Originally Posted by Jadelin View Post
from what I've heard about CO2 it dissolves MUCH less readily in water than Oxygen does. So I think that the water movement that helps get oxygen in the water is much less effective at getting CO2 in the water. In a low tech situation I don't know if water movement would cause a net gain or net loss of CO2, but it doesn't seem like it would have a very strong effect either way.
Yes great wording, this is what im looking for whether bubbles/surface agitation, causes a net gain or loss of CO2, in a non Co2 injected planted aquarium.
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post #7 of 21 (permalink) Old 09-12-2011, 07:13 PM
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I've done some research, and at first it only made me more confused than before.

Many sites say that CO2 is actually much more soluble in water than O2 is; around 30 TIMES more.
http://www.engineeringtoolbox.com/ga...er-d_1148.html
http://wiki.answers.com/Q/Which_gas_...lity_CO2_or_O2

So I really couldn't think why is is that surface movement can increase O2 but decreases CO2. My guess was that it has something to do with the fact that there is a much greater amount of O2 in our atmosphere than CO2; more than 500 times as much. (http://en.wikipedia.org/wiki/Earth%2...re#Composition)

But then I realized that in a case where we've pumped an artificial concentration of CO2 into our tank's water, with more surface contact that extra CO2 will be released into the atmosphere, because the concentration is much higher than the balance would be otherwise.

From what I can gather, normal CO2 levels in surface water is >10 ppm. So if you're trying to get a higher ppm of CO2 than that, more contact with the atmosphere through surface disturbance will cause the CO2 to move from the area of high concentration to low concentration and it will degas out of your water. Naturally, pressure will make a difference in how much CO2 can dissolve in your water; I believe that the closer to sea level the greater the pressure and the higher the CO2 level can be.

So if we follow this idea, if the plants in your tank has been using the CO2 in your water and you now have a concentration of CO2 that is less then the equilibrium concentration, it seems like higher surface disturbance would indeed cause the atmospheric CO2 to have a chance to dissolve into your water because your water has less CO2 than it is capable of holding at that temperature and pressure.

So from what I can gather the short answer is: yes in a non-CO2 injected tank with healthy growing plants that are using CO2, increased surface agitation and water movement should 'increase' the level of CO2 in the water by keeping that level constant instead of it being depleted by the plants.

Incidentally, I would assume it is the same principle in affect when we add O2 to the water; the fish are using the O2 and therefore there is less in the water than is possible, and adding agitation lets the atmospheric O2 dissolve into the aquarium water faster. Since the percentage of O2 in the atmosphere is so much higher than that of CO2, the equilibrium saturation of O2 in water is going to be higher, even though CO2 is actually more soluble.

Hopefully someone can let us know if I'm wrong about all this, but it seems like I'm probably on the right track, since it all seems to make sense (but who knows?).

As a side note
Tap water does tend to have lots of CO2 dissolved in it (maybe because it's under pressure?), however without having a constant steady supply that CO2 is not going to stick around long in your tank. In fact constant flushes of CO2 followed by levels of low CO2 when it leaves can cause BBA to get a foothold in low tech tanks because of the fluctuating CO2 levels. For this reason it's actually better to do less frequent water changes in a tank like this and/or let the water sit out to degas before adding it if possible.

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post #8 of 21 (permalink) Old 09-12-2011, 07:18 PM
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from what I understand.. dissolved CO2 in the water (baseline) is maybe around 5 ppm.. this is a result of just plain old contact with the atmosphere in the first place.. injecting bubbles may increase this minimally but nothing significantly, mostly because the percentage CO2 in the atmosphere is just not that high. other gasses will bond more quickly and leave less room for the CO2.
this may be wrong or wrongly interpreted.
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post #9 of 21 (permalink) Old 09-12-2011, 07:53 PM
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C02 equalizes between 2-3ppm in most no c02 injected tanks tanks. some will argue as low as .5 ppm
c02 will not diffuse frm the airstone because the nitrogen gas bubble it is riding on is rising so fast there is no time for adequate diffusion of either c02 or o2.. the key is the surface agitation it creates.. which if is good should keep you around 4-8 ppm of o2 and 2-3 ppm of c02 which under low light situations and good fertilizers can grow many plants just fine

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post #10 of 21 (permalink) Old 09-12-2011, 07:54 PM Thread Starter
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Quote:
Originally Posted by Jadelin View Post
if the plants in your tank has been using the CO2 in your water and you now have a concentration of CO2 that is less then the equilibrium concentration, it seems like higher surface disturbance would indeed cause the atmospheric CO2 to have a chance to dissolve into your water because your water has less CO2 than it is capable of holding at that temperature and pressure.

Incidentally, I would assume it is the same principle in affect when we add O2 to the water; the fish are using the O2 and therefore there is less in the water than is possible, and adding agitation lets the atmospheric O2 dissolve into the aquarium water faster. Since the percentage of O2 in the atmosphere is so much higher than that of CO2, the equilibrium saturation of O2 in water is going to be higher, even though CO2 is actually more soluble.
yes exactly, that is the principal I was hoping to take advantage of Co2 flowing from an area of higher concentration(the air) to an area of lower concentration(the tank)

but i have also thought that (in a lightly stocked) heavily planted tank, because of the plants photosynthesizing and producing oxygen perhaps bubbles won't increase oxygen saturation, but rather decrease it instead.


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post #11 of 21 (permalink) Old 09-12-2011, 08:10 PM
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Originally Posted by SeaSlug182 View Post
but i have also thought that (in a lightly stocked) heavily planted tank, because of the plants photosynthesizing and producing oxygen perhaps bubbles won't increase oxygen saturation, but rather decrease it instead.
It seems to me that if your dissoloved oxygen is so high surface agitation is actually decreasing it you have more than adequate O2 and don't have to worry about it. I don't know if that's even possible, but if it were I don't see how it would cause any problems.

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post #12 of 21 (permalink) Old 09-12-2011, 08:12 PM
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Originally Posted by HD Blazingwolf View Post
the key is the surface agitation it creates.. which if is good should keep you around 4-8 ppm of o2 and 2-3 ppm of c02 which under low light situations and good fertilizers can grow many plants just fine
I agree with this one. If you are not injecting CO2 into your tank, then the surface agitation caused by an airstone will perk a little CO2 into the water. Not much, but a little.
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post #13 of 21 (permalink) Old 09-12-2011, 08:59 PM
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CO2 dissolves very readily into water, and comes out of solution equally readily. Oxygen, by contrast doesn't dissolve very easily into water. In a tank with plants, the plants will be using CO2 from the water to a limited extent, reducing the amount of dissolved CO2. But, the water is in contact with more CO2 in the air at the water surface, so CO2 will be dissolving into the water from the air, very likely faster than the plants are using it. Agitating the water surface increases the surface area, and causes water near the surface to circulate more, both of which will help increase the rate of dissolving of CO2 from the air into the water. It also increases the rate of dissolving of oxygen into the water from the air. It is that surface turbulence that is caused by an air bubbler that makes it effective, both as a way of getting rid of CO2 from the water and of getting some CO2 and oxygen from the air into the water.

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post #14 of 21 (permalink) Old 09-13-2011, 12:25 AM Thread Starter
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first, sorry about my half finished post at 12:56 I was about to be late for math class
and second, Thanks for clarifying Hoppy, great post as usual


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post #15 of 21 (permalink) Old 09-13-2011, 12:40 AM
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A few people have touched on this, but let me restate for clarity's sake - Bubbles themselves, even pure oxygen bubbles, would do very little to increase the O2 in the tank. The same is true for CO2. The exception is if we're talking about *tiny* bubbles, like the ones created by a good diffuser. Smaller bubbles create more total surface area versus larger bubbles containing the same total amount of injected gas (I did the math a long time ago - surface area of a sphere and whatnot). In addition, smaller bubbles ride around in the tank for much longer.

The benefit of a bubbler is not to forcibly inject O2 and/or CO2 back into the tank - its purpose is to agitate the water's surface, thus increasing the surface area (waves have a much larger surface area than a plane) and speeding up the equalization process.

And yes, in a low-tech tank, surface agitation is a good thing.
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