# CO2 in aquarium and atmosphere



## Ray (10 Feb 2009)

I probably learnt this at school, so please excuse the dummy question.  Why, if we have nearly 400ppm CO2 in the atmosphere (see: http://www.guardian.co.uk/environment/2008/may/13/carbonemissions.climatechange), do we need to administer CO2 to the aquarium to arrive at 30ppm?  Why does it disperse into the atmosphere where the concentration is much higher than our tank?

Thank you.


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## GreenNeedle (10 Feb 2009)

because the natural amount of CO2 that passes into the water and stays there is in the region of 3-5ppm (maybe slightly wrong) and therefore we add more to try and maintain a higher amount.

This is why to keep stable in non CO2 setups water changes can be a bad thing (CO2 wise) because the tap water will have more in it and will make the level rise until it gasses off.  Even letting it gas off and then add the natural ppm water will mean when you add it to the tank you draw more in.

It is god though contrary to many beliefs (and indeed mine until recently) to keep the water turbulent as this natural level will then be maintained by the turbulence drawing in the natural level 

Non CO2 tanks kept are therefore run with lower 'controlled' light with minimal ferts and decent flow/turbulence with water changes every 3,6,12months or never.  Not weekly like an injected tank.

AC


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## scottturnbull (10 Feb 2009)

It would be interesting to force air through a diffuser, and see what ppm results. A normal air-pump probably wouldn't work. It might need a high-pressure air-compressor. I think you can buy them for Aeroponics. But even then, the oxygen might cause too much turbulence, and degas the water.

If there was a simpler way of adding CO2, no doubt someone would be using it already.


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## Ray (10 Feb 2009)

Exactly Scott - its interesting that we have 5ppm in the water and 400ppm in the air yet we are forced to buy CO2, presumably because of the relative insolubility of gases in water compared to air.  But in concentration terms there is so much more in the atmosphere - no wonder emersed growth is so much better even after pumping nutrients and water up from the roots.  No wonder high tech tanks get 10x growth.

Andy, I agree about the turbulence in low tech tanks.  My 25l test tank got excellent results with a HOB filter (super high aeration in the filter, outflow and ripples right down the tank) and easy carbo - at least it could maintain 5ppm CO2 and also good oxygen for bacteria to work on ammonia etc.  I'm quite interested to see how far you can go without CO2 - with just liquid carbon, disappointing that George abandoned his recent non CO2 tank....


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## GreenNeedle (10 Feb 2009)

I would speculate that under the same flow conditions that a tank using liquid CO2 like EasyCarbo/Excel would fare almost exactly the same as injecting Co2 from a bottle.  You would need to measure it somehow though to ensure you had the same C supply with each.

With the non CO2 method I used to assume that it was a case of there only being a little amount a still surface would be best to 'retain' what little there was.

It was suggested to me by the last person I would ever want to prove me wrong that it was better to have a turbulent surface.  That peeved me but I admitted my error 

When you come to think of it the plants are still using the CO2 and much like O with a still water surface then the gaseous exchange is less so while the plants are using the '5ppm' it is depleting.  When the surface is more turbulent the gaseous exchange helps replenish the 'equilibrium' and therefore retains the level at nearer 5ppm.

This of course isn't possible with injection because we are adding more than the 'natural' amount and therefore a turbulent surface will not bring in more CO2 because the water already has more than 'equilibrium' thus losing CO2 'gassing off' and trying to return to the 5ppm.  We counteract this by injecting more to compensate the extra gassing so we can maintain a good surface ripple (or at least I do. lol)

AC


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## ceg4048 (11 Feb 2009)

PPM - 101

Think about a 1 liter bottle of air and compare the mass to a bottle filled with water. There are many more "parts" in the water bottle and more "millions of parts" because of the waters higher density so you cannot compare part per million directly with a bottle of air.

The amount of one fluid (the solute) that dissolves in another (the solvent) depends on the pressure of that solute within the solvent. If I cover the bottle of air I will only be able to dissolve more CO2 in that air if I increase the pressure of the CO2 and the pressure of the air. If I then uncover the bottle both the pressurized air and CO2 will escape until the pressure of the air/CO2 mixture is the same as that of the atmospheric pressure. If I cover the bottle and climb a mountain and then uncover the bottle, then immediately more air and CO2 will escape from the bottle until the pressure in the bottle is equal to the new (lower) atmospheric pressure. This is called (unsurprisingly) the equilibrium pressure. No movement of CO2 or any other gas can move from atmosphere to the bottle unless either the pressure in the bottle is reduced or the pressure in the atmosphere is increase. 

So the amount of molecules moving from atmosphere to water is based on the barometric pressure. If the barometric pressure rises then CO2 moves across the waters surface into the tank and if the barometric pressure decreases then CO2 moves out of the water because it's pressure is higher than that of the atmosphere. Since there are many more molecules of water than there are molecules of air the calculation of ppm will have a different value. 400ppm of CO2 in air cannot have the same value in ppm in water directly. The calculation "ppm" is really number of CO2 molecules divided by the number of water molecules or, number of CO2 molecules divided by number of air molecules. 

The value 1 part per million in water means that for every single molecule of CO2 in the bottle there are 1,000,000 molecules of water. So, for a given rise in CO2 pressure, a certain number of CO2 molecules will enter the water but since there are many more water molecules than air molecules then the distribution of CO2 molecules is "thinner" in water than they would otherwise be in air.

This is why CO2 is injected into the tank under pressure. So if your needle valve reads higher than zero this allows more CO2 to dissolve in the water than can dissolve from the atmosphere alone. The higher the needle valve reading the higher the CO2 pressure and the more gas dissolves. However, since the dissolved gas is at a higher pressure than the CO2 in the atmosphere the CO2 moves across the surface into the atmosphere attempting to reach equilibrium (or, the same pressure) with that of the atmosphere.

Sending compressed air through a diffuser does nothing beneficial, firstly because the mass ratio of CO2 is very low (400 molecules of CO2 per 1,000,000 molecules of air) and secondly because the disturbance created by the bubbles breaking the surface drives what CO2 is available out of solution in exactly the same way as the shaking of a fizzzy drink bottle drives off the CO2 and makes the drink go flat.

In a non-CO2 environment, as the plants absorbs CO2, it's pressure within the water drops and more CO2 in the atmosphere which is now at a higher pressure moves across and into the water to replace what was depleted - unless there are bubbles or surface disturbance which drive off what is held by the water. There is a race between what is absorbed and what is depleted by agitation. Agitation normally wins.

Liquid carbon products do not have the same effectiveness or efficiency as CO2 gas because they are not 100% CO2. There are a complex aldehyde which must be metabolized and converted to CO2. Therefore the CO2 yield at the end of the conversion cycle is no where near that of 100% CO2.

Hope this makes sense...

Cheers,


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## Ray (11 Feb 2009)

Nice to see Clive posting a mini lecture - I've missed that lately  .  That does make things clearer!  So I think you are saying that there are more water molecules per given volume so 400ppm in air translates to 5ppm in water due to the higher density of water molecules?  Atmospheric pressure, temperature and so forth also have a bearing but its not terribly important to the home aquarist and his 5ppm.



			
				ceg4048 said:
			
		

> This is why CO2 is injected into the tank under pressure. So if your needle valve reads higher than zero this allows more CO2 to dissolve in the water than can dissolve from the atmosphere alone. The higher the needle valve reading the higher the CO2 pressure and the more gas dissolves. However, since the dissolved gas is at a higher pressure than the CO2 in the atmosphere the CO2 moves across the surface into the atmosphere attempting to reach equilibrium (or, the same pressure) with that of the atmosphere.



This suggests the pressure in my gas cylinder is dissolving the CO2?  I'm not so sure about that - my understanding is that the pressure enables the CO2 to bubble out through the water in spite of the water pressure.  I think we just bubble it through the water and it dissolves by diffusion - migrating to areas of lower concentration and eventually the net movement is out of the tank through the surface?



			
				ceg4048 said:
			
		

> In a non-CO2 environment, as the plants absorbs CO2, it's pressure within the water drops and more CO2 in the atmosphere which is now at a higher pressure moves across and into the water to replace what was depleted - unless there are bubbles or surface disturbance which drive off what is held by the water. There is a race between what is absorbed and what is depleted by agitation. Agitation normally wins.



OK, so as Andy suggests in a non CO2 tank surface agitation is good to replace CO2 depleted by the plants, in a high tech tank its bad (at least during the lighting cycle) as it speeds up the loss of CO2.



			
				SuperColey1 said:
			
		

> It was suggested to me by the last person I would ever want to prove me wrong that it was better to have a turbulent surface.


That would be the Mrs then?


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## GreenNeedle (11 Feb 2009)

Ray said:
			
		

> my understanding is that the pressure enables the CO2 to bubble out through the water in spite of the water pressure.  I think we just bubble it through the water and it dissolves by diffusion - migrating to areas of lower concentration and eventually the net movement is out of the tank through the surface?



This is true in a way but not the whole truth.  Even with no diffuser the bubble that shoots to the surface will diffuse a little on it's way.  We try and reduce this big bubble to microbubbles as they don't rise as quickly and are easier to hold in the water for longer.  Ladder diffusers have been proven pretty efficient recently according to another thread on this very forum somewhere.



> OK, so as Andy suggests in a non CO2 tank surface agitation is good to replace CO2 depleted by the plants, in a high tech tank its bad (at least during the lighting cycle) as it speeds up the loss of CO2.



Whilst being 'bad for CO2' it is good for O2 and therefore means you can afford to push a little harder with the CO2 whilst making sure the O is fine for fish.  At night it helps keep the O up while the plants consume it 



> That would be the Mrs then?


Nope one of the phosphate haters on TFF 

AC


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## ceg4048 (11 Feb 2009)

SuperColey1 said:
			
		

> Ray said:
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Yep, the cylinder pressure accomplishes two things. At the surface of the diffuser the gas pressure must be higher than the water pressure otherwise there is no movement of the bubbles into the water. That's why when the gas is shut off and the CO2 line becomes unpressurized the higher water pressure causes movement of water into the diffuser. That's why we need a check valve to halt this reversal. Secondly, as Andy says, the bubbles emerge at a higher pressure and this forces the diffusion into the water at that higher pressure. 

The high concentration at the diffuser site causes movement of the dissolved CO2 away from that site but that is caused by what now becomes osmotic pressure, but it was the higher barometric pressure that forced the gas into solution in the first place. The longer we can maintain that high pressure bubble in the water, the more CO2 dissolves. Ladder type diffuser are less elegant but do a better job than the prettier disc type diffusers which simply eject the bubbles straight up. in smaller tanks this is easily overcome by increasing the injected pressure and thus increasing the diffusion rate since the volume of water is low. In larger tanks this becomes much more difficult.

A counter flow external reactor like the AM500/100 or the DIY types like the ones Ed Seeley and others make do a really good job because they maximize the contact time of bubble within the reactor and there is extra contact time as the remaining bubbles travel through the filter return line.

Cheers,


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## a1Matt (11 Feb 2009)

Ray said:
			
		

> Nice to see Clive posting a mini lecture - I've missed that lately  .



Hear hear   It is great to see a post that I can learn something from, even if I do need to read it a few times to understand it   



			
				SuperColey1 said:
			
		

> ceg4048 said:
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Having recently gone from FE CO2 to no CO2 I have been contemplating increasing my surface agitation, as I had a feeling it would improve my Co2 exchange into the tank.  So this post makes me happy, I will stop contemplating and just do it now!  

Thanks guys.


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