How fast does CO2 gas off?

[Sanniejop]

I am a bit confused. I am setting up a new aquarium (~400L netto) with a CO2 system. During a first test to see if I could get the CO2 level high enough with the equipment I have, I observed the following.
Over a period of 9 hours the drop checker turned yellow. So I was pleased with that result. But after that point I switched off the CO2 and watched the drop checker to turn back green and blue again.
But to my surprise over a period of 24 hours I didn't saw any change in color? I though I maybe killed the drop checker by to high CO2 but I found out that this is not possible.
Only after 32 hours I saw the checker turning back a little bit greener. I understand the drop checker has a slow response time (lag time) but I didn't expect it to take so long to get back to green.
I am really surprised by this slow rate of gassing of the CO2. Is this normal?
I have very low surface movement, but I have a big surface skimmer, so the surface of the water is very clean, so I expected a faster and more effective gas off rate.
On the other hand this made me think I can use probably a much lower CO2 bubble rate then during my test, which makes it very economic.

But on the other hand I am worried about the Oxygen concentration in the tank. If the off gassing of CO2 goes so slow caused by the still surface, how will the oxygenation of the water be affected?
Or will the plants just generate so much oxygen that the fish will survive the night?
When the water is saturated with oxygen will this than also gas off to the air, or will it be stable, or forced out by the CO2 dosage?

I was planning to leave the CO2 running day and night, just like I do on my other 175L tank, which currently works well for plants and fish.

 

[Little]

In my tank, the drop checker turns from dark green to blue after three hours of the co2 turning off. I believe 1 hour of that is the reaction time. I have two air driven sponge filters so I assume mine degassed faster but I also use a reactor so that will hold some gas’s longer too.

It also takes three hours to go from blue to dark green from when the co2 turns on.
Again I think it has about an hours lag for the drop checker reaction plus the co2 reactor I have takes a little while to fill.

May be worth doing what’s called a PH profile. You basically check your ph frequently from co2 on to see how long and how far it drops and if it remains stable.
Some advocate to aim for up to a 1 drop in PH and for it to happen in a few hours and maintain stable through the co2 period. I do half of this as am medium tech / medium energy.

 

[Sanniejop]

May be worth doing what’s called a PH profile. You basically check your ph frequently from co2 on to see how long and how far it drops and if it remains stable.
Some advocate to aim for up to a 1 drop in PH and for it to happen in a few hours and maintain stable through the co2 period. I do half of this as am medium tech / medium energy.

Yes, I have read about it. But currently I don't have a PH pen so I can not do this.
I like your approach of medium tech. I am also aiming for that, needing less maintenance.

 

[Wookii]

I have very low surface movement

This is the reason you've experienced very slow off-gasing. Decent surface agitation (Ideally breaking of the water surface) and continual off-gasing of CO2 is critical for stable CO2 during your injection period. It may seem counterintuitive when you are trying to forcibly add CO2 to the water column, but without the high level of surface agitation you get a situation where CO2 levels will continue to increase throughout the injection period as you saw with your yellow drop checker (which should be avoided). The surface agitation essentially provides a ceiling to rising CO2 levels, which is what you want, such that levels rise in the first 2-3 hours after injection starts, then they hit your target level, and stay there stable until the gas is turned off towards the end of the photo period.

You should aim for your drop checker to hit green within 2-3 hours of the gas being switched on, and then staying there. If the colour continues to change, your injection rate is too high, and/or your surface agitation too low.

Even with good surface agitation though, you'll not usually see your drop checker go back to blue. I've never had a tank where the drop checker went back to anything other than dark green by the start of the next injection period. You have to use fairly extreme methods to get all the CO2 out of the water column within half a day, such as using large airstones etc, but it isn't necessary, residual levels after the photo-period are fine as long as surface agitation is keeping O2 levels up and continues to reduce CO2 levels during lights out, and will mean you hit target CO2 levels easier the next day.

on the other hand I am worried about the Oxygen concentration in the tank

. . . and you are correct, oxygen is critical to tank health, and is a second important reason why decent surface agitation is critical, particularly during the latter end of the lights out period where plants are no longer releasing O2.

I was planning to leave the CO2 running day and night

That's unnecessary to be honest, and really is just a waste of relatively expensive CO2. None of the CO2 injected overnight is being used by the system, and is just putting fish under unnecessary strain since CO2 levels will elevate even higher at night whilst the plants aren't consuming it, at a time when O2 levels are at their lowest also. Just use a plug in timer to turn the solenoid off over night.

 

[dw1305]

Hi all,
Edit: what @Wookii says.

On the other hand this made me think I can use probably a much lower CO2 bubble rate then during my test, which makes it very economic.

I think so. Yellow would just indicate you had a lot of CO2, but it wouldn't differentiate the pH once all the bromothymol blue had <"become protonated"> (it is a <"narrow range pH indicator">- above pH 7.6 it is blue (deprotonated) and any acidity below pH 6, yellow). So you might have had a <"lot of dissolved CO2">, and a very low pH value, when the CO2 went off. CO2 is highly soluble.

I though I maybe killed the drop checker by to high CO2 but I found out that this is not possible.

That's right, no moving bits in the drop checker and nothing to break.

I am worried about the Oxygen concentration in the tank. If the off gassing of CO2 goes so slow caused by the still surface, how will the oxygenation of the water be affected?

Potentially the same issue with low oxygen levels and a still surface.

Or will the plants just generate so much oxygen that the fish will survive the night?

Unfortunately back to <"unknown unknowns">. Because maintaining a high level of DO <"is so vitally important"> I just wouldn't take that risk.

cheers Darrel

 

[Sanniejop]

Potentially the same issue with low oxygen levels and a still surface

My aquarium has a build in skimmer, about 15 cm wide. Surface water is falling down about 4 cm deep into a chamber. I assume that the surface of the water is "broken" in this process. Would this be enough to guarantee enough oxigenation of the water?
I am a physicist and have an understanding about liquids properties, viscosity, surface tension and so on but i dont have a goof idea what i must think of when people talk about breaking the surface?
The surface will always be water molecules. So I would expect that the uptake of oxigen will be the same when the surface is just kept really well clean (e.g. with a surface skimmer) Indeed, if the water is totaly standing still the distribution of oxigen in the whole aquarium is dependend of concentration gradients and diffusion rates. Diffusion into the water wil slowdown very rapidly because of the layer high oxigen concentration created at the surface. But the water is always moving quite a bit, but without creating a ripple on the water.

I understand the concept of creating a big CO2 leak by high surface movement and than using a strong CO2 input device. That way you can get in a short time a certain stable CO2 value. But I think it is consuming much more CO2 than stricly needed. Also the tank will experience much quicker ph and CO2 concentration variations. Is that something that plants and fish like?
I dont know how big the variations will be with a lower continueos CO2 injection. But the speed of variations will atleast be slower i guess?

Just my thoughts
Maybe I have to invest into a ph pen.

 

[dw1305]

Hi all,

Surface water is falling down about 4 cm deep into a chamber. I assume that the surface of the water is "broken" in this process. Would this be enough to guarantee enough oxigenation of the water?

You would have thought so.

I am a physicist and have an understanding about liquids properties, viscosity, surface tension and so on

So we are interested in the effective gas exchange surface area and Henry's law.

of when people talk about breaking the surface?

Indeed, if the water is totaly standing still the distribution of oxigen in the whole aquarium is dependend of concentration gradients and diffusion rates. Diffusion into the water wil slowdown very rapidly because of the layer high oxigen concentration created at the surface. But the water is always moving quite a bit, but without creating a ripple on the water.

I think high tech most people have a visible ripple, but <"laminar flow would also work">.

I understand the concept of creating a big CO2 leak by high surface movement and than using a strong CO2 input device. That way you can get in a short time a certain stable CO2 value. But I think it is consuming much more CO2 than stricly needed. Also the tank will experience much quicker ph and CO2 concentration variations. Is that something that plants and fish like?
I dont know how big the variations will be with a lower continueos CO2 injection. But the speed of variations will atleast be slower i guess?

I might just try turning down the CO2 bubble rate and see if the drop checker returns to blue more quickly. I'm not a CO2 user, so I have no practical experience of CO2 injection.

cheers Darrel

 

[Sanniejop]

So we are interested in the effective gas exchange surface area and Henry's law.

Any idea if it makes a difference if you have an open top or a closed lid on top? I would think the partial co2 pressure is not that different. The lid is not hermetical closed.

 

[Connswater]

hermetical closed

But does moist, relatively still, warm air, not 'absorb' CO2 more slowly than dry, cooler, moving air from a body of water? I'm not a scientist, but using a cover glass seems to significantly reduce the amount of CO2 needed.

 

[Andy Pierce]

Any idea if it makes a difference if you have an open top or a closed lid on top? I would think the partial co2 pressure is not that different. The lid is not hermetical closed.

It makes a huge difference. I think @hax47 did a nice job of quantifying this. CO2 is more dense (1.98 g/L) than air (1.29 g/L) so if you have a closed top lid the bubbled out CO2 will layer over the water surface and give you a massive surface area for CO2 exchange. With an open top stray air currents in the room will disperse the CO2 layer.

 

[dw1305]

Hi all,

It makes a huge difference. I think @hax47 did a nice job of quantifying this. CO2 is more dense (1.98 g/L) than air (1.29 g/L) so if you have a closed top lid the bubbled out CO2 will layer over the water surface and give you a massive surface area for CO2 exchange. With an open top stray air currents in the room will disperse the CO2 layer.

Any idea if it makes a difference if you have an open top or a closed lid on top? I would think the partial co2 pressure is not that different. The lid is not hermetical closed.

I think @Andy Pierce is right. Personally I'm more interested in dissolved oxygen than CO2, and I want to out gas any "extra" CO2.

Cheers Darrel

 

[Andy Pierce]

Personally I'm more interested in dissolved oxygen than CO2, and I want to out gas any "extra" CO2.

I understand the concern but don't think you have to worry. The drop checker is your friend here. If the drop checker says you're at 30ppm dissolved CO2 then effectively your tank is equilibrated to an atmosphere of 20,000ppm CO2 in air (vs the usual 400-600ppm). You still have over 200,000ppm oxygen in air so there's lots of O2 to be had. It can go wrong in places if your tank is not in equilibrium, for example if water circulation is poor, so water movement is important.

 

[Connswater]

equilibrated to an atmosphere of 20,000ppm CO2 in air (vs the usual 400-600ppm). You still have over 200,000ppm oxygen in air

Great post, and though I don't aim for 30 ppm, I find 20 more than adequate, I am sure you are spot on, sadly I don't understand the 20,000 ppm aspect of your post, my fault, not your fault. I didn't study chemistry at the examination level, all those decades ago.

 

[dw1305]

Hi all,

I am sure you are spot on, sadly I don't understand the 20,000 ppm aspect of your post.....

It is the amount of atmospheric CO2 you would need to give you an equilibrium value of 30 ppm dissolved CO2 in the tank water.

This is because CO2 is <"highly soluble in water">, so what controls dissolved CO2 levels is the level of atmospheric CO2.

Cheers Darrel

 

[Andy Pierce]

You can check here for some discussion of how a steady-state amount of CO2 dissolved in water relates to the effective local atmospheric concentration of CO2, water alkalinity and water pH.

To the original question about an aquarium with a lid, in that case the local atmosphere is the headspace between the water surface and the lid and that local atmosphere can be quite different from the overall atmosphere of the room the aquarium is in.

 

[Sanniejop]

I understand the concern but don't think you have to worry. The drop checker is your friend here. If the drop checker says you're at 30ppm dissolved CO2 then effectively your tank is equilibrated to an atmosphere of 20,000ppm CO2 in air (vs the usual 400-600ppm). You still have over 200,000ppm oxygen in air so there's lots of O2 to be had. It can go wrong in places if your tank is not in equilibrium, for example if water circulation is poor, so water movement is important.

Now I understand why water movement, and especially water surface movement is so important.
To distribute the oxygen getting into the water at the interface, through the whole tank.
And to prevent trapping of a lot of CO2 in the tank water with a CO2 cushion on top of the water at the interface. I had almost no ripple on the water surface.

Is it important to have also a good airflow above the the water? To prevent a CO2 cushion above the water?
My tank is a room divider and the aquarium builder has made only one opening in the hood at the wall side and no other ventilation grills or something in the hood.
Would it be wise to create some?

Btw, here some pics of my new tank, now five weeks old. Putting over plants from my small old tank. Just passed the diatom phase and thread algae phase.

 

[dw1305]

Hi all,

Now I understand why water movement, and especially water surface movement is so important.
To distribute the oxygen getting into the water at the interface, through the whole tank.

That is it. As @Andy Pierce says, oxygen is ~20% of the atmosphere, but it isn't very soluble, so it takes a long time for the entire water volume to become saturated with oxygen, particularly if you have a small gas exchange surface area to water volume ratio. <"You can use a surface ripple"> to increase the effective gas exchange surface area and if you add in laminar flow to turbulence? You have increased that gas exchange surface area again.

Other ways of mitigating for low flow include adding dissolved oxygen to the water column (<"photosynthesising plants"> are fantastic for this) or reducing the <"Biochemical Oxygen Demand (BOD)"> of the water.

And to prevent trapping of a lot of CO2 in the tank water with a CO2 cushion on top of the water at the interface. I had almost no ripple on the water surface.

The rate that carbon dioxide (CO2) will out-gas from the water's surface is dependent upon the steepness of the gradient between atmospheric and the aquariums CO2 level. I think @Andy Pierce is saying that we don't need to worry too much about this because, if you inject 30 ppm CO2 (<"mimic an atmosphere with 20,000 ppm CO2">) you have a pretty steep gradient with the 424 - 600 ppm CO2 in the atmosphere <"Trends in CO2 - NOAA Global Monitoring Laboratory">. This means that, as long as there is some ventilation, CO2 will escape to the atmosphere.

You can think of <"opening a can or bottle of carbonated drinks">, that CO2 really "wants" to out-gas.

In waste water work, where you can effluents with a very high BOD that you are treating aerobically, you need to make sure <"you efficiently out-gas CO2"> from confined spaces, as <"CO2 pooling"> poses a real risk to human health.

cheers Darrel

 

[Andy Pierce]

You don't necessarily need good airflow above the water, but you need something predictable (either high or low) so you can set your CO2 injection to the right level. If you have a lot of atmospheric mixing you will need to inject more CO2 into the water column. If you have reasonably tightly covered lit setup, you will need to inject less (maybe a lot less). Another way to get oxygen into the system (and some water movement) is to bubble air in through an airstone - I do this and like it but the aesthetic is not to everyone's taste. In a covered lid set-up the airstone ensures at least some minimum level of atmospheric mixing into the above-the-water-surface headspace.

In terms of water mixing, I think laminar flow is just fine, particularly with efficient flow at the surface that then descends through the tank. I have never really bought into the "surface ripples are good" concept because I don't think it meaningfully increases the surface area for gas exchange, and more-so because you can see surface ripples and think everything's all super, when in fact you might just have a very choppy shallow layer at the surface and no deep water mixing at all.

 

[dw1305]

Hi all,

Another way to get oxygen into the system (and some water movement) is to bubble air in through an airstone - I do this and like it but the aesthetic is not to everyone's taste

<"I use a venturi">, it works well and I don't mind bubbles.

In a covered lid set-up the airstone ensures at least some minimum level of atmospheric mixing into the above-the-water-surface headspace.

If you have a well fitting lid you would need air-stone, rather than a venturi.

In terms of water mixing, I think laminar flow is just fine, particularly with efficient flow at the surface that then descends through the tank. I have never really bought into the "surface ripples are good" concept because I don't think it meaningfully increases the surface area for gas exchange, and more-so because you can see surface ripples and think everything's all super, when in fact you might just have a very choppy shallow layer at the surface and no deep water mixing at all.

That is my understanding as well.

cheers Darrel

 

[PeerUnk]

<"I use a venturi">, it works well and I don't mind bubbles

I did mind the bubbles, splashing and noise, so I built this thing, and a bigger version for my display tank:
(Edit: picture of big version included)
The muffled air comes out on top, and water without air on the right. A Czech muffled airlift…

I even have a version as a closed loop with K1 micro as a filter. The K1 goes up on the air lift side, and goes down down at the back. The are holes smaller than the K1 to keep the K1 in the loop and allow water to go in and out.

Both work great, lots of aeration and flow and a minimum of noise and air bubbles.