# CO2 injection versus "air" or aeration



## mudboots (Jun 24, 2009)

When I was a kid I had two powerheads on a fish-only tank and they had air inlet tubes, which resulted in lots of tiny bubbles. The purpose as I recall was to keep the water alive in a manner of speaking.

In planted tanks many folks inject CO2, some don't, but I never notice posts from those who have air inlets, or let their filter outflows aerate the water otherwise. 

I played around with a sump setup on a small tank where there was significant aeration due to the wet/dry type flow typically associated with sumps. I had pearling on some plants, but I also had a lot of organic carbon from decomposing plant matter that I purposefully loaded the tank with (this was a filtration test).

So does anyone have any experience with aeration or even air injection (via something similar to the powerhead/air tube thing) in a planted tank that can be used to compare with CO2 injection???

I'm really curious about this. I mean, the air that plants grow in is "air", and plants I see in nature growing under water are not "CO2 injected" by any means. So why do we inject CO2 rather than "air"?

Keep in mind I have never dosed CO2 in a tank before...


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## BruceF (Aug 5, 2011)

I use a venturi on a powerhead at times. My assumption is always that plants can do pretty well in atmospheric levels of co2 and that is what I assume I pump into the tank, air. I never can figure out if the plants are pearling or the bubbles are getting caught up in the leaves. I find it helps with algae also. Though I do get tired of the noise. I also periodically do large water changes and let the filters run for 10 minutes or so. Once a week if I am ambitious. 
I’ve never used co2 so I have no idea what the differences are.


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## Tikulila (Feb 18, 2011)

I'm a lot less experianced than you two, but I do have an assumption/theory.
1st off, lets start at from what the air is made of:
Nitrogen, N2 78.08%
Oxygen, O2 20.95%
Argon, Ar 0.93%
Carbon dioxide, CO2 0.033%
and some noble gasses, O3 (ozone) and more

So, say that 0.033% of CO2 go in, BUT, if I'm not wrong (and correct me) there is also a problem of osmosis, meaning after we get a certain amount of CO2 into the water, osmosis will kick in.So we got a very small amount of it in the water, but so little it's used up really fast. In the air you have 0.033%, but the volume of the air is so big, that it's ALOT of CO2.

And of course, the fish, who also add CO2 to the water.

If anyone has anything to add, do it and correct me if I'm wrong .

Oh and, this is only a Theory.


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## Michael (Jul 20, 2010)

In tanks that are heavily aerated, CO2 levels in the water will always be in equilibrium with atmospheric CO2. This level will be lower than in tanks with supplemental CO2, and possibly lower than tanks with the respiration of many organisms--bacteria, animals, and plants that are not photosynthesizing.

Aeration and atmospheric CO2 equilibrium also means that a brightly lit tank will never become completely depleted of CO2 by intense photosynthetic activity.

In Walstad tanks, the theory is that one should avoid heavy aeration so that any respiratory CO2 in excess of atmospheric equilibrium will not be lost. In practice I suspect that this is pretty difficult to manage, and I don't try to do it very hard. I don't aerate my tanks deliberately, but I also don't obsess about possible CO2 loss from HOB filters and the like. My guess is that my tanks stay near atmospheric equilibrium all the time.


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## UltraBlue (Mar 8, 2011)

There is a limit to the amount of dissolved CO2 you can get from the air, using air you cannot get to the amounts required by high light driven tanks. This has to do with partial pressure of the gas to be dissolved and temperature. Since we all operate our tanks within a narrow range of pressure and temp, there is a limit to the amount of CO2 that will be dissolved from the air. Natural systems that have high levels of CO2 do not get it from the air, rather, they are fed by aquifers with a mineral composition that chemically adds CO2 to the water, leading to CO2 rich natural systems. I don't remember the exact make up that creates this and I am only regurgitating what another more knowledgeable person than I has said on high CO2 natural systems.

In other words, in order to reach the 25 ppm's and up needed, an artificial source must be added.

To Tikulila's point, I don't think osmosis is what you are looking for. An equilibrium is reached where the amount degassed is equal to the amount dissolved. Osmosis refers to the movement of fluids via molecular forces, not dissolved gases. Diffusion applies to dissolved gases, however isn't applicable in this case.

Here is a reference source that covers CO2 entering a water column within sea water, but the ideas apply to our smaller systems.

Look particularly at "What causes water column CO2 to change"

http://www.ozcoasts.gov.au/indicators/water_column_partial_pressure.jsp


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## BruceF (Aug 5, 2011)

Still the question is how much co2 can I get from the air? I realize it is less than I can get from pure CO2, and that it may not be enough to grow plants at hyper-growth rates. 
I suspect that using a venturi I am in fact adding co2, oxegen and probably nitrogen. I don't really see why I would be driving the co2 off. The atmosphere is 350ppm. (and climbing)http://co2now.org/

{please don't mistake me for someone who understands these things!}


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## agutt (Jan 28, 2009)

BruceF said:


> Still the question is how much co2 can I get from the air? I realize it is less than I can get from pure CO2, and that it may not be enough to grow plants at hyper-growth rates.
> I suspect that using a venturi I am in fact adding co2, oxegen and probably nitrogen. I don't really see why I would be driving the co2 off. The atmosphere is 350ppm. (and climbing)http://co2now.org/
> 
> {please don't mistake me for someone who understands these things!}


I think the bottom line here is that in order to have a significant amount of CO2 in your tank (enough to see a difference) you would have to diffuse pure CO2...

What kind of plants are you trying to grow?


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## agutt (Jan 28, 2009)

Also, in nature plants do not grow in a contained environment as they do in our tanks...


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## BruceF (Aug 5, 2011)

I am probably growing 20 different plants right now. Some of them grow very slowly but most of them do fine.


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## agutt (Jan 28, 2009)

When it comes to CO2 dosing, and again this is all in my opinion, it is kind of like getting leather seats in a car... you don't need it to drive the car, but it makes it look better. CO2 will quicken your growth and provide you with lush plants, but many other things factor in such as lighting and fert dosing... The three of these things all correlate, and getting them balanced is what separates the amateur tanks from the "pro" looking ones. 

To answer your original question, aquatic plants in nature have a much more developed ecosystem due to the fact that they are actually in nature. The ecosystems are also much much much bigger... more water, much much much more light, and more nutrients that are constantly replenished. They are conditions that are much harder to regulate on a smaller scale, and that must be much much more accurate. Lastly, we plant aquariums so we can enjoy how they look, which is most definitely not the purpose of these plants in nature...

All that being said... If you are growing stem plants and/or lots of very high light plants, C02 is a must in my opinion. Right now my tank has no C02, but my plant demands dont need it (Crypts and mosses)... best thing you can do is experiment and see what works best for your needs


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## UltraBlue (Mar 8, 2011)

Hi BruceF,

to answer your question, I think the equilibrium for CO2 into water at sea level and around that is 7 ppm. Just as an FYI, nitrogen doesn't dissolve into water  Aeration is a more effective way to add O2, which at good levels is at low ppm's. 6-8 ppm, if memory serves me right.

You have to remember that when using the venturi, the exchange of gases goes both ways, you add CO2 but also lose it when the boundary layer of the water is broken that is why an equilibrium is struck. When injecting CO2 from a pressurized source you are breaking the boundary layer with pure CO2 and the exchange is unidirectional, it is not when we talk about atmospheric air. The injected CO2 is still lost to the atmosphere at the boundary layer at a rate dependent upon the movement of the surface water exposed to the air that is why we must continuously add CO2 when injecting it, we replace what is used by the plants and what is lost to the atmosphere. 

Some plants will not grow without added CO2, many will do fine with lower Co2 levels, lower light, and good flow. 

Natural systems cannot be compared to ours.


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## mudboots (Jun 24, 2009)

Good conversations...I definitely agree with the basic ideas that it's something folks ought to experiment with and that it all depends on what you're growing, and why you're growing it.

I am starting to think back the 125 I used to have. It was a non-CO2 tank with little movement in the water. I can't help but wonder how it'd have looked had I blown a bunch of bubbles in there with about 1500 gph movement...maybe something I can consider in the future.


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## queijoman (Jun 23, 2008)

agutt said:


> To answer your original question, aquatic plants in nature have a much more developed ecosystem due to the fact that they are actually in nature.


I'll add also that many of the plants we attempt to grow submerged grow partially emersed in nature or only in very particular conditions.


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## Tikulila (Feb 18, 2011)

First, an interesting article (the one which I could understand and didn't have weird formulas): http://www.hallman.org/plant/huebert.html
I will try to add aeration to my tank to check, but I also have DIY CO2.

Will anyone else try, so will will have a few results?


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## BruceF (Aug 5, 2011)

Great article Tilulila. I guess the problem is how do you tell what is going on?


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## BruceF (Aug 5, 2011)

I think this might fit in here somewhere...
Even with the recent rise in CO2, there's still over five hundred times as much oxygen around. The difference in aquatic systems is less, because almost thirty times more CO2 than oxygen dissolves in a given amount of water. But CO2 is still less concentrated in water than is O2 if the water has equilibrated with the air above-500 divided by 30, or a mere 17 times more oxygen. 
[Steven Vogel. The Life of a Leaf pg 90.]


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## Yo-han (Oct 15, 2010)

I think most important to understand is where most of the CO2 in our (non CO2 injected) tanks (and in nature) comes from, not the atmosphere, but from bacteria. Most bacteria use O2 and produce CO2. In nature there are a lot of bacteria in the substrate and lots of dead leaves in the water as food to be used to produce CO2. Dead leaf (carbon) + O2 (waterfall + osmosis) = CO2. CO2 in nature is higher than one might expect in some places. In our tanks, the bacteria in the filter and substrate are quite low compared to the number of plants, so CO2 is used up faster (especially under high light). Our unnatural high density of fish add extra CO2 to the water and with a short lighting period, or even better a siesta during the lighting period, our fish and bacteria can produce enough CO2 for the plants. If you would leave dead matter in your tank, this would produce even higher CO2. (warning also, higher ammonia and nitrite, so this is not a recommondation).

So the amount of CO2 rises during the night and decreases during the day. O2 the other way around. When adding CO2 the entire day and night, O2 would be lowest in the morning (all has been used by bacteria and fish) and CO2 the highest. This is why I (and so does ADA to make it more convincing) do inject CO2 only during the day and aerate! the tank at night. Especially the last thing made a huge difference in my tank since I do that. By adding extra oxygen during the night, bacteria work harder (and die less!). I've no longer a surface scum layer anymore and my aquarium produces way more nitrate since. And I feel like my water is more clear, but this could be subjective.

CO2 injection versus "air" or aeration: I say both! This way you take care of your plants, fish AND bacteria!


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## Cavan Allen (Jul 22, 2004)

Yo-han said:


> So the amount of CO2 rises during the night and decreases during the day. O2 the other way around. When adding CO2 the entire day and night, O2 would be lowest in the morning (all has been used by bacteria and fish) and CO2 the highest. This is why I (and so does ADA to make it more convincing) do inject CO2 only during the day and aerate! the tank at night. Especially the last thing made a huge difference in my tank since I do that. By adding extra oxygen during the night, bacteria work harder (and die less!). I've no longer a surface scum layer anymore and my aquarium produces way more nitrate since. And I feel like my water is more clear, but this could be subjective.
> 
> CO2 injection versus "air" or aeration: I say both! This way you take care of your plants, fish AND bacteria!


I have started doing the same, and I've also noticed some big improvements. I did also start adding Exel and turned up the co2 a tiny bit, but... I think it will be what helps me finally get rid of the cyano bacteria I've had. No more surface scum, and plant growth has really picked up. We will see.


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## BruceF (Aug 5, 2011)

Makes me wonder if the cyanobacteria aren't advantaged by the anaerobic processes.

Reminds me of this thread. 
http://www.aquaticplantcentral.com/...1-biological-filtration-translated-dutch.html


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