O2 vs CO2 concentration - The Planted Tank Forum
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post #1 of 27 (permalink) Old 01-07-2014, 06:05 AM Thread Starter
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O2 vs CO2 concentration

There has been something I have been pondering about. It has been stated that the more O2 in you inject the more CO2 you can inject. In theory this makes some sense. What we do know is CO2 makes your pH go down and O2 makes it go up. Let me say I am not trying to change my pH. It has always been a rule of mine to work with what you got as your livestock needs stability. My question is if you inject O2 with CO2 how much does this actually change the CO2 saturation in the water? I would tend to think O2 would dilute I guess you could say the CO2 concentration that is present and they would just battle each other. If that is the case then where would the advantage be?

Now the one thing I can say is I have been able to run my CO2 more when I have injected O2 so is that where the advantage is and not with the actual concentration between the O2 an CO2.

Hopefully this makes sense, I tried to explain it the best I knew how. There is a little experiment I did and saw some good results so that is why I was curious.

-Thanks!


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post #2 of 27 (permalink) Old 01-07-2014, 12:57 PM
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If you inject O2, the CO2 concentration won't change and vise versa. Both CO2 and O2 can range from 0-100% saturation independent from each other. If you're an odd ball like me and are interested in learning more about gas diffusion check out these you tube videos. He does a great job explaining these laws.
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post #3 of 27 (permalink) Old 01-07-2014, 01:26 PM
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O2 does not make pH go up. It's the act of "oxygenating" water that drives CO2 out of water that causes said pH to rise, not the O2 itself.

From what I understand, increased CO2 concentrations reduce the ability of fish to take in available oxygen, so I'd assume there's some limitation on adding CO2, regardless of O2 levels.

Are you injecting O2 via a pressurized tank?


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post #4 of 27 (permalink) Old 01-08-2014, 02:02 AM
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Originally Posted by Chronados View Post
O2 does not make pH go up. It's the act of "oxygenating" water that drives CO2 out of water that causes said pH to rise, not the O2 itself.

From what I understand, increased CO2 concentrations reduce the ability of fish to take in available oxygen, so I'd assume there's some limitation on adding CO2, regardless of O2 levels.

Are you injecting O2 via a pressurized tank?
I think it might have a lot to do with the fact that most people running co2 don't have as much surface agitation as people running a setup without.In my tank I've found that I can turn the co2 up to roughly 60 ppm with lots of surface agitation and the fish are all fine but at 25-30 ppm with little or no surface agitation the fish struggle. This makes me want to believe that in most cases of inexperienced hobbyist Saying they gassed their fish it was actually their fish dying of hypoxia. Sorry if I got off topic but I think that is where the belief that o2 and co2 levels are linked stems from.
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post #5 of 27 (permalink) Old 01-08-2014, 02:12 AM Thread Starter
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Originally Posted by Zorfox View Post
If you inject O2, the CO2 concentration won't change and vise versa. Both CO2 and O2 can range from 0-100% saturation independent from each other. If you're an odd ball like me and are interested in learning more about gas diffusion check out these you tube videos. He does a great job explaining these laws.
Thanks I will check these videos out. So if the saturation of each is independent of each other then in theory you should be able to inject CO2 to the target values without any ill effects to the fish or the CO2 saturation if you're "oxygenating" the water? Would that be correct? Just something I have been thinking about if there is a way to have both work together.




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Originally Posted by Chronados View Post
O2 does not make pH go up. It's the act of "oxygenating" water that drives CO2 out of water that causes said pH to rise, not the O2 itself.
I think you might have misunderstood the point I was trying to make in my question through simplified means as an example.

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From what I understand, increased CO2 concentrations reduce the ability of fish to take in available oxygen, so I'd assume there's some limitation on adding CO2, regardless of O2 levels.
Yes this is true and their is a limitation on adding CO2 because of this as it causes ill effects to the fish. Hence the question what effects of "oxygenating" the water have on the concentration of current CO2 in the water. If both can coincide together to make it possible to have the nesscary saturation of CO2 while still being able to cater to the demands of the livestock and not gassing them out.

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Originally Posted by Chronados View Post
Are you injecting O2 via a pressurized tank?
No, air pump


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post #6 of 27 (permalink) Old 01-08-2014, 02:22 AM
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Originally Posted by Texan78 View Post
Thanks I will check these videos out. So if the saturation of each is independent of each other then in theory you should be able to inject CO2 to the target values without any ill effects to the fish or the CO2 saturation if you're "oxygenating" the water? Would that be correct? Just something I have been thinking about if there is a way to have both work together.

Hence the question what effects of "oxygenating" the water have on the concentration of current CO2 in the water
It depends on what you mean by "oxygenating" the water. Actually just straight up injecting O2? No, no effect whatsoever on CO2. Air pump and a bubbler? CO2 levels will drop from the increased surface agitation. And no, even though CO2 and O2 concentrations are not linked, high CO2 concentrations impair the ability of gills to absorb O2, hence why you can't just inject CO2 to saturation, regardless of the amount of O2 in the water.

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Originally Posted by Texan78 View Post
No, air pump
You're not really injecting O2 in the same sense as CO2 then, you're just bringing it back to/closer to atmospheric levels. Imo, you're far better off just ensuring good plant growth (which puts excess oxygen, ie past atmospheric levels, in the tank, as evidenced by pearling). CO2 concentrations at the levels that the vast majority of planted tanks need are absolutely fine for fish. Sure, you can add an air pump too, but you'll have to add that much more CO2 to ensure the same levels.


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post #7 of 27 (permalink) Old 01-08-2014, 02:38 AM
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  • CO2 and O2 are independent of each other
  • CO2 has a much faster diffusion rate than O2
  • O2 is temperature dependent on the water
  • As temperature rise gases leave the system...couple examples-pop going flat when left out (CO2 leaves the system), boiling water and tasting it (less oxygen so tastes different)
  • You can achieve higher than 100% saturation of oxygen...say 120% when photosynthesis is going on because of the slower diffusion rates.
  • The issue with fish is acidosis-"In high concentrations, CO2 can block the respiration of CO2 from the fishes gills and cause oxygen starvation. Since the gills depend on a concentration differential between CO2 levels in the blood and the water to transfer gases, high CO2 levels in the water will reduce the amount of CO2 that can be transferred from the blood. Also, high levels of CO2 in the blood will cause acidosis"-(Taken from thekrib.com) This is why higher O2 levels allow higher CO2 levels...concentration gradients.
  • Fick's first law of diffusion


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post #8 of 27 (permalink) Old 01-08-2014, 02:43 AM Thread Starter
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Well crap, there goes that theory...LoL

Sounds like regardless how much saturation of oxygen you have the CO2 will prevent the fish from using that O2.

Was this suppose to read "In high concentrations, CO2 can block the respiration of O2 from the fishes gills and cause oxygen starvation."?


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post #9 of 27 (permalink) Old 01-08-2014, 02:51 AM
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Quote:
Originally Posted by Texan78 View Post
Well crap, there goes that theory...LoL

Sounds like regardless how much saturation of oxygen you have the CO2 will prevent the fish from using that O2.

Was this suppose to read "In high concentrations, CO2 can block the respiration of O2 from the fishes gills and cause oxygen starvation."?
No, its correct how it was written. Fish take in water through the gills, convert/takes out oxygen and breath out CO2. If the water has a higher concentration of CO2 it is hard for the fish to get rid of the CO2 in it's body (concentration gradient). If it can not get the CO2 out than the CO2 concentration in its blood will increase...causing acidosis...death.

Edit-Works the same way with oxygen. The more oxygen in the water the more the fish can take in to offset the pH swing in it's blood.


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Last edited by UDGags; 01-08-2014 at 02:54 AM. Reason: spelling
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post #10 of 27 (permalink) Old 01-08-2014, 03:04 AM Thread Starter
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Ah very true, forgot all about that, makes sense.

So will oxygenating the water have any benefit for the fish when the CO2 is on?

Reason I am curious and you would be familiar with this since you have an Apex too. I have my CO2 set to turn off at 6.50 and defer for 30 mins to allow it to come up. Anything lower and it is to much for the fish. Just using the pH as a gauge and not for CO2 saturation before anyone confuses that. at 6.50 it also keeps it roughly within the 1 point swing. Now before when I would just let it come up on its own in at 30 mins it wouldn't come up much. I then added air to come on when the CO2 is off. I noticed during this experiment this allowed my CO2 to run much longer intervals than without the air. So got me thinking, does the oxygenating of the water change the saturation of the CO2. One would think it would since the pH raises but not sure. But since higher O2 levels allow higher CO2 levels they it would appear they could coincide allowing plants to get what they need without harming the fish but that doesn't seem the case or is it?


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post #11 of 27 (permalink) Old 01-08-2014, 03:13 AM
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Another point is that gills are VERY effective at aqueous gas transfer for O2. But CO2 concentrations need to be higher in the blood than the water for them to export the CO2 out of their system.

I suppose some active transport system would be great, but I'm not aware if that is the case or not. Some have air bladders and some remain inactive(behavior changes) when the CO2 is high in natural systems which reduces the CO2 accumulation and increases the O2. Many catfish do this.

Few add pure O2 gas to the aquarium. I tried it for a time, mostly to see if algae would decline, nope, it did quite well at 200%. Another added headache and cost, we already have the plants adding O2 during the light cycle, so another 1-2 ppm, is added via the plant growth, in some dense natural plant beds, this can be 5-8 ppm extra. This is toxic, so O2 is MUCH more toxic to aquatic life than CO2 actually. If you are off by 5 ppm +/- with O2, that's not good enough. Few use it generally, but I suspect many would kill their fish faster.

Aeration is a better choice, but that's done only at night, or you can convert to a wet/dry which adds another 1-2 ppm O2 vs a canister filter over the entire 24 hr cycle.




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post #12 of 27 (permalink) Old 01-08-2014, 03:37 AM
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Quote:
Originally Posted by Texan78 View Post
Ah very true, forgot all about that, makes sense.

So will oxygenating the water have any benefit for the fish when the CO2 is on?

Reason I am curious and you would be familiar with this since you have an Apex too. I have my CO2 set to turn off at 6.50 and defer for 30 mins to allow it to come up. Anything lower and it is to much for the fish. Just using the pH as a gauge and not for CO2 saturation before anyone confuses that. at 6.50 it also keeps it roughly within the 1 point swing. Now before when I would just let it come up on its own in at 30 mins it wouldn't come up much. I then added air to come on when the CO2 is off. I noticed during this experiment this allowed my CO2 to run much longer intervals than without the air. So got me thinking, does the oxygenating of the water change the saturation of the CO2. One would think it would since the pH raises but not sure. But since higher O2 levels allow higher CO2 levels they it would appear they could coincide allowing plants to get what they need without harming the fish but that doesn't seem the case or is it?
Depends how you oxygenate. My guess is if you have a bubbler/air stone than you would have more surface agitation and probably losing more CO2 since its diffusion is higher. This is why you see the pH difference.

Tom referred to the wet/dry versus canister. The wet/dry allows more O2 in the water and if you seal the system you won't lose much CO2 (back to concentration gradients ).

I personally followed Tom's and Gerry's system using the wet/dry and sealing it. I have no air stones.


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post #13 of 27 (permalink) Old 01-08-2014, 04:09 AM Thread Starter
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Aeration is a better choice
That's actually what I am doing when I say injecting O2. I probably just worded it wrong. Injecting O2 via means of aeration.

The CO2 comes on hr before the lights come on and goes off an hr before they go off. The air pump is on an anti-sync schedule meaning anytime the CO2 is the air is switched on. I have noticed since doing that I am able to run the CO2 in longer intervals.

This is just an example and these numbers have no true meaning, just for example purposes only.

Let's say my CO2 is on and and for example we will say it is at 30 ppm. pH gets to 6.50 and shuts off for 30 mins. Air comes on during that time. Let's say because of oxygenating it causes he CO2 ppm to come down 5 ppm. But because of where my pH is at since my CO2 is triggered off the pH probe the Apex will allow it to run longer allowing it to inject more CO2 in to the tank. So the O2 may lower the CO2 saturation but the CO2 can now run longer and lets say injecting 7 ppm in it's next interval before it gets to 6.5 pH. I guess what I am trying to find out is if the O2, oxygenating, aerating what ever you want to call it will remove more CO2 than what is injected.

I hope this makes sense, I know it is not the best description. It makes sense in my head just having a hard time explaining it.


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post #14 of 27 (permalink) Old 01-08-2014, 05:02 AM
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Another point is that acid irritates fish's gills which produce mucous to protect the gills from being burned. Excessive mucous then prevents efficient gas exchange and leads to problems. CO2 also inhibits hemoglobin which reduces the amount of oxygen a fish's blood can transport.

I wrote a little bit about it back in 2008 while looking through an animal physiology text book.
http://www.aquaticplantcentral.com/f...tml#post394113
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post #15 of 27 (permalink) Old 01-08-2014, 02:54 PM
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I guess what I am trying to find out is if the O2, oxygenating, aerating what ever you want to call it will remove more CO2 than what is injected.
It depends on a few factors. I'll try and explain the basics below.



In the image above the green circles represent CO2 molecules in our imaginary tank. The number of green circles on either side of the water surface wants to remain in equilibrium. The number of green circles that are dissolved into water depends on a few things but for explanations sake let’s assume an equal ratio even though that’s not real life. Therefore, the image above is said to be in equilibrium.




If we inject more green circles into the water like the image above then the circles will want to leave the water since we have a lot more in the water. Remember, these gases want to stay in equilibrium.

Notice in the second image there are less circles near the surface? The molecules are closer to the surface so it’s easier for them to “pop out” into the air. Is there anything you can think of to increase that rate?

What if we made the water surface larger, the tank depth more shallow or pumped the molecules closer to the surface. That would increase the rate of green circles leaving the water correct?

Increasing surface agitation moves molecules closer to the surface allowing them to leave quicker than no water current. If we trickled water over a huge surface area, such as a wet/dry filter, this would allow more molecules to leave the water because we have a much larger surface to air percentage.

These are some of the factors that will influence how much CO2 we have in the water. If we increase surface agitation or surface area we lose CO2. However, carbon dioxide molecules are not the only ones we deal with. The oxygen molecules will also follow these same principles.

If we have fish “eating” oxygen molecules in the water they eventually become depleted causing the fish to suffocate and die. So we provide water movement or other means to bring oxygen molecules back into the water. These principles work both ways. Remember the gas(s) wants to remain in equilibrium.

Do you see why CO2 and Oxygen are independent now? What about the balance of CO2 injection rate and water movement? That is a VERY brief explanation of the laws I posted earlier and how they can affect what we are trying to do.

Last edited by Zorfox; 01-08-2014 at 03:42 PM. Reason: wording
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