[steve1]

I am an aquarium fan.  CO2 is important to plants.  My question is, all other things being equal, would a fishtank contain more dissolved CO2 with or without air being passed constantly through the water through what is called an airstone?

The answer will potentially help a lot of people.  Thanks.

[Arkcon]

Most aquarium websites and books will remind you, that the bubbles from the airstone add trivial gas exchange.  Instead the disturbance of the surface of the water adds more, say from the output of an external filter.  Also, air contains only traces of CO₂, so the effect is even more negligible.  The fish in the tank provide CO₂ plants need.  If you need to culture just plants alone, you can try adding more, say by capturing the gas release from vinegar on baking soda in a jar, and piping that gas into the tank.  Again, with the caveat, there is negligible transfer.

[steve1]

Yes, you're right.  People who use compressed CO2 get up to 30 ppm.  Non-pressurized CO2 fish tanks, depending on water conditions, get around 3 ppm.  But CO2 above 30 ppm can be deadly to fish.

Anyway, the other thing you're right about is that the bubbles not contributing much CO2 to the water.  They don't.  I think it's the increase in water surface area that does it, but we're talking about maybe 2% of the total.

I'm really trying to get to the bottom of this.  CO2 builds up from respiration (O2 in, CO2 out) of fish, plants, and most bacteria to a level above equilibrium.

During photosynthesis, CO2 is taken up by plants and oxygen released into the water.  Plants can deplete the CO2 level in less than 8 hours, the length of my photoperiod.

Thanks for the help.

[Intanjir]

Whether an airstone contributes significantly to gas equilibration appears to be one of those eternally debated internet subjects.
I think this guy raises some good points in favor:
http://briansaquariumcare.com/airstones.html

How do you derive the 2% estimate?

I would think that increased equilibration (however it might be achieved ) could easily affect CO2 either way depending on how many animals/bacteria and how many actively growing plants are in the tank. If the plants are already full-size then they don't net use as much CO2. Really this is all more of a biology question.

[curiouscat]

If you guys think the surface absorption is what matters why not use a surface wheel / paddle? That should give better surface transfer than a bubbler I think.

[billnotgatez]

Would a fountain approach expose more water to air?

@steve1
Could you tell us if you are asking about
a plant only aquarium or animal / plant aquarium

[steve1]

I'm talking about an aquarium with live plants and fish.

My question was really would an airstone/bubbler increase the rate that CO2 comes out of solution from the water?  Thinking about gas exchange at surface of bubbles and increased water to atmosphere surface area, and times during a 24 hour period when dissolved CO2 is higher than equilibrium level.

There are some people who think that more gas exchange will let in CO2, which should only happen when the CO2 level in the tank is below equilibrium level.

[Intanjir]

The equilibrium level is key.
CO2 is rather hydrophilic compared to other gases, so its equilibrium concentration in water will be higher than another gas for the same partial pressure.

Wiki tells me that the Henry's Law constant for CO2 is 0.034 M/atm.
...and it tell me that the partial pressure of CO2 in the atmosphere generally is 0.000387 atm(and rising!,[hehe])
so the equilibrium concentration is around 0.000013 M.

[curiouscat]

...and it tell me that the partial pressure of CO2 in the atmosphere generally is 0.000387 atm(and rising!,[hehe])
so the equilibrium concentration is around 0.000013 M.

~0.5 ppm?

[Intanjir]

Molarity of water is about 55 Moles/liter so I am getting ~0.25 ppm.   Feels low, but I think it's correct.

If it is correct, and a typical aquarium ecosystem has more like 3ppm dissolved CO2, then increased rate of gas exchange with air will remove CO2 directly.

But still will it result in more or less CO2 concentration?

Animals/Bacteria are pretty good CO2 producers. If 3 ppm and 0.25 ppm are right then it is clear that they are the dominant source of CO2. As such the rate at which they produce CO2 is of principal importance. They need O2 to produce CO2. It is conceivable that although increased gas exchange directly removes CO2 it nevertheless results in an increased amount of CO2 because it increases the O2 concentration. How sensitive is this biological CO2 production on the O2 concentration?

[steve1]

Mmm... the bit about adding Oxygen causing CO2 concentration to fall, well, I think it's a big question for a lot of AHs, but I don't think it decreases the amount of dissolved CO2.  I think Henry's Law says something about this, but I can't remember the exact idea and application of the equation.  I've been trying to understand it.

Seems like if you add Oxygen, you just increase the total pressure and the partial pressure of O2 in the water.  Like i said, this is a big question but I don't think the added O2 displaces any CO2.  Thanks for your patience.

Oh, by the way, I can't reallly make an educated guess at how many bacteria are in a 10 gallon tank, but there are a lot.  Since bacteria are very small, there may be room on surfaces for hundreds of thousands of them, probably more like millions.

[Intanjir]

I don't think anyone brought up the possibility of O2 displacing CO2 until you did just now.

I was suggesting that O2 would act as a reagent for the production of new CO2, thus potentially increasing the CO2 concentration.

[steve1]

Sorry.  Don't remember exactly what I was thinking of.

I don't know where the carbon would come from.

[Intanjir]

The plants/algae presumably give off a fair amount of carbon compounds the bacteria can eat.
Also you feed the fish their carbon and much of it will get eaten by the bacteria.

You could probably feed the bacteria directly too(dash of sugar?) if you wanted even more CO2.
Might be easier than directly bubbling CO2 gas.

[steve1]

That's what a lot of people do - it's called "hi-tech".  You have to fertilize in that case too.  Nitrogen, Phosphorus, Potassium, and traces.  I'm doing low-tech, no extra CO2.