CO2 Off-Gassing

I read a lot about CO2 quickly off-gassing in tanks with high surface agitation.

And statements like:

.........
Air pumps will gas off the CO2 you add in the water almost instantly. This would waste the CO2 you are injecting. It is not necessary to run one at night either in an aquarium with adequate flow and surface agitation.

Which certainly don't match my experience.

So I'd like to ask one question.
How long does it take for the PH in your aquarium to reset to the non-CO2 state after it goes off at night.

Here's my example.

Aquarium PH7.

C02 on at 6am.
Lights ramp up at 9:30am (PH6)
Photo Period 10:30am to 5pm (PH5.8)
CO2 off at 1pm
Lights ramp down 5pm to 6:30pm (PH5.9)

Air stone comes on at 4pm to 5am to assist with off-gassing.

I'm lucky if PH=7 before 6am.

You will note that I'm not even injecting CO2 during the majority of the active photo period.

Fast CO2 Off-Gassing seems like a myth to me.

 

I read a lot about CO2 quickly off-gassing in tanks with high surface agitation.

And statements like:



Which certainly don't match my experience.

So I'd like to ask one question.
How long does it take for the PH in your aquarium to reset to the non-CO2 state after it goes off at night.

Here's my example.

Aquarium PH7.

C02 on at 6am.
Lights ramp up at 9:30am (PH6)
Photo Period 10:30am to 5pm (PH5.8)
CO2 off at 1pm
Lights ramp down 5pm to 6:30pm (PH5.9)

Air stone comes on at 4pm to 5am to assist with off-gassing.

I'm lucky if PH=7 before 6am.

You will note that I'm not even injecting CO2 during the majority of the active photo period.

Fast CO2 Off-Gassing seems like a myth to me.

This question depends on a lot of variables including how powerful your air pump is, how much existing surface agitation there is, how much CO2 is being injected, etc.

For an example, I was helping a client redo their CO2 system on their hightech 125g with a sump. They had tons of areas to off gas CO2 with their outflow creating tons of surface agitation, and a large sump with many air stones. They could not get a CO2 concentration high enough for successful plant growth and were plagued with BBA and staghorn algae. They were injecting so much CO2 that it was impossible to count. Fish were unphased and pH would barely drop.

As a remedy, we removed all air stones from the sump. We also significantly reduced the amount of surface agitation(but not entirely because it is important).

Within hours, we had to turn down the CO2 level because the fish had started to gasp at the surface.

With all of their air stones, it was simply wasting the CO2. In this case, and many others like it, fast off gassing is not really a myth.

I’d recommend watching this video on CO2 by Dennis Wong:

It is not necessary or efficient to run air stones while the CO2 is being injected. You can do it, but you’ll need significantly more CO2 to achieve the same results.

In my own hightech display tank with a Good amount of surface agitation and a surface skimmer, the pH goes from the Low/mid 6s to the mid 7s just about 4-5 hours after my lights turn off. This measurement was taken around 6 months ago and I haven’t tested since so I might have to take a measurement soon again to verify.

 

Not a myth, it just depends on surface area and agitation.
Here's the written version of the above video, it doesn't strictly answer your question because it depends on the individual tank, but maybe some of the topics discussed will help.


Although I am curious as to why you are turning your CO2 off so many hours before lights off?
And do you use a drop checker?

 

Great article -Mak-

Here's a quote from the article that gives a basis as to why I don't recommend running an airstone with CO2 running:
"There is a balance being having good gaseous exchange - and having too aggressive gaseous exchange that prevents CO2 levels from building up meaningfully at all."

In my experience, airstones are too aggressive with the gaseous exchange.

 

Thank you both. It's a good video and write up.

I'm sure I've seen it before, but it was interesting to go over it again.

I'd say my set up falls somewhere between example A and example B, in terms of surface agitation and circulation.

My tank is 36x18x18 inches (50g) so pretty much ideal dimensions for off gassing.

I run a Fluval 407, that cycles the tank 7.5 per hour. With a three foot tank that should be plenty.

My surface agitation looks to my eye exactly the same as that of Dennis Wong's in the video.

I also have a surface skimmer.

Vishaquatics - I use the air stone to assist getting the C02 out of the tank before the following days cycle, and to ensure sufficient oxygen levels at night.

-Mak- - I'm turning the C02 off during the photo period because A) I know my PH will stay at my target 5.8 for the rest of the photo period. B) To assist in getting back to PH7 by 6am the next morning.

I haven't used a drop checker in several months, as my PH probe is far more accurate in monitoring minute by minute changes in the tanks PH.

I've just taken a quick video to try to show the surface agitation. Let me know if your thoughts.

 

Circulating water from the bottom of the tank to the top basically does nothing. I guess whoever made the video never bothered testing this.

Surface agitation is fairly ineffective method of gas exchange.

Air stones are very effective and gas exchange with bubbles works both ways so they're probably also very effective at removing CO2 from a tank. I don't think I'd run one when lights and CO2 are on.

 

Thank you both. It's a good video and write up.

I'm sure I've seen it before, but it was interesting to go over it again.

I'd say my set up falls somewhere between example A and example B, in terms of surface agitation and circulation.

My tank is 36x18x18 inches (50g) so pretty much ideal dimensions for off gassing.

I run a Fluval 407, that cycles the tank 7.5 per hour. With a three foot tank that should be plenty.

My surface agitation looks to my eye exactly the same as that of Dennis Wong's in the video.

I also have a surface skimmer.

Vishaquatics - I use the air stone to assist getting the C02 out of the tank before the following days cycle, and to ensure sufficient oxygen levels at night.

-Mak- - I'm turning the C02 off during the photo period because A) I know my PH will stay at my target 5.8 for the rest of the photo period. B) To assist in getting back to PH7 by 6am the next morning.

I haven't used a drop checker in several months, as my PH probe is far more accurate in monitoring minute by minute changes in the tanks PH.

I've just taken a quick video to try to show the surface agitation. Let me know if your thoughts.

What I don't get is why your pH is still so low after a night of running an airstone. Does the pH rise fairly quickly in the beginning, or is it a slow rise all the way until morning?

Circulating water from the bottom of the tank to the top basically does nothing. I guess whoever made the video never bothered testing this.

Surface agitation is fairly ineffective method of gas exchange.

Air stones are very effective and gas exchange with bubbles works both ways so they're probably also very effective at removing CO2 from a tank. I don't think I'd run one when lights and CO2 are on.

Out of curiosity, what's the reason for this and what would be a better method of gas exchange? And what then, makes air stones more effective?

 

Out of curiosity, what's the reason for this and what would be a better method of gas exchange? And what then, makes air stones more effective?

It's probably due to water pressure enhancing gas diffusion (in both directions).

 

What I don't get is why your pH is still so low after a night of running an airstone. Does the pH rise fairly quickly in the beginning, or is it a slow rise all the way until morning?

My PH recovery is always slow. Around 0.04 per hour without the air stone, and around 0.07 per hour with the air stone.

Can't tell you why.

Contributing factors could include:
Water temperature of 28.4c
Low KH (anything from 1-2)
Low GH (anything from 2-4)