Degassing of CO2 in the planted aquarium - some thoughts

[Art]

An area of confusion for me has been the degassing of CO2 in my aquarium. It's often talked about but we don't seem to have real data except for that done by our friend @Dennis Wong in his article How to measure CO2 levels in an aquarium.

According to this article, standing water sits at .6 ppm of dissolved CO2 and a normal planted aquarium will degass to 2-3 ppm. Therefore, our tanks are never full degassing CO2, especially because we supplement to such a high level. This, in turn, concerned me of an ever increasing CO2 level.

For example, if we started at 2-3 ppm and we went up to 30 ppm on day one, is it possible that on day two you degass only to 3-4 ppm and that is your starting point. Adding the same amount of CO2 would then end up at 31 ppm on day two. This would continue increasing the total high over time.

Curious that I have not created a CO2 disaster in my tanks in the past, I decided to pull up my Apex Fusion log showing my pH drop daily. Here it is:


I was pleasantly surprised to see that my 1.3 - 1.4 pH drop is absolutely consistent day after day. This seems to indicate that I am not on an ever increasing cycle but one that repeats exactly the same day after day.

Does this mean that degassing really isn't an issue to be concerned about or that it's more about oxygenation than decreasing CO2?

 

[Count Krunk]

Confirms that in your tank it certainly isn't an issue.

I've never heard of a co2 build up like it's calcium or something.

 

[Dennis Wong]

Most CO2 injected aquariums have between 3-8ppm of CO2 after overnight with no CO2 injection. The important thing is that the pH reading (in a CO2 injected system) used to measure a 1 pH drop, cannot be based on the tank water left over night as said water will still contain significant amounts of CO2. A standing glass of CO2 saturated water takes 2 days-ish to degas to near equilibrium. Leaving a glass standing for 24 hours can still have residual 5ppm of CO2 (a 1 pH drop from this point will give approx 50ppm of CO2).

Water from tank left to stand in a beaker for 24hours with no stirring/agitation.

The correct way to determine the baseline pH is to manually degas a water sample by shaking or stirring for some time.

This means that most folks doing a 1 pH drop from overnight pH value in tank water are injecting much higher amounts of CO2 during their light window. Around 50-70+ppm in most cases. Depending on the type of fish & available oxygen levels, this may or may not be an issue. Cardinals tetra seem fine at 70ppm of CO2.


By extension, its a myth that most fish's upper limit is 30 or 35ppm. I couldn't find a fish species that had any issues in the 40-50ppm range. Barr, who also had accurate test equipment, commented that Discus had issues at 40ppm and Discus is known to be a CO2 sensitive fish. For most common/cheap smaller tetras, they seem extremely resilient towards CO2 levels of 40-50ppm.



I think tanks have to over-inject quite a bit to hit the fish - this isn't hard to do in tanks with poor gaseous exchange (oily film on water, etc).

 

[Naturescapes_Rocco]

Hey Dennis, so great to see you here! I've read every article of yours multiple times over.

I was wondering if you have any new/updated thoughts about the Hanna CO2 test kit? I've been using it quite frequently but without something like the Oxyguard it's been hard to know if it's as accurate as it feels.

In this article, you wrote:

Of the CO2 test kits we tested, the Hanna CO2 test kit HI3818 gives very accurate readings compared to the Oxyguard CO2 analyser (+-5ppm). The Hanna CO2 kit is only 40+ USD, which makes it very affordable for what it does. The main drawback is that the kit requires titration and some minor maths.

At the moment we are still testing it to see if other pH buffers in the water column significantly affect its readings.

Have you been able to do other tests, or do you have other thoughts? Would love to hear from you!

 

[Dennis Wong]

The hanna kit seems to work well, with the caveat that depending on how vigorously you swirl the test as it is done, there can be some variance. i guess this is no different from other test kits that need shaking of reagents in a certain manner. It seems to work on seiryu tanks (5-6 dKH ?) on a test I saw in the office, matching oxyguard numbers. However, I have not yet tested it on even more alkaline water (say 18+ dKH).

 

[Naturescapes_Rocco]

Do you still have the oxyguard? I'd love to see a test in more alkaline water. What about in 0dKH?

 

[Art]

This means that most folks doing a 1 pH drop from overnight pH value in tank water are injecting much higher amounts of CO2 during their light window. Around 50-70+ppm in most cases. Depending on the type of fish & available oxygen levels, this may or may not be an issue. Cardinals tetra seem fine at 70ppm of CO2.

So this is what I understood from your article, @Dennis Wong. So I still feel like I'm missing something.

Here is how I run my tank:

I have an Apex pH Controller that tracks pH but will only turn off CO2 if the pH drops below 6.0. I start CO2 at 9 am and it runs through the cycle. You can see the drop in the graph above. The controller isn't letting the pH get below 6 so you see it bouncing at that number. I then turn off CO2 overnight and it starts again the next morning. The pH rises to over 7.4.

If I understand your statement, I am starting CO2 the next day into water that already has plenty of CO2 and the system pushes another 1.3 degrees to pH 6. This would result in CO2 concentrations much more than 30 ppm even though pH is exactly the same every day. Correct?

If so, I guess it is working for me because I haven't gassed my fish but it does show that things aren't always what you think they are.

 

[Dennis Wong]

So this is what I understood from your article, @Dennis Wong. So I still feel like I'm missing something.

Here is how I run my tank:

I have an Apex pH Controller that tracks pH but will only turn off CO2 if the pH drops below 6.0. I start CO2 at 9 am and it runs through the cycle. You can see the drop in the graph above. The controller isn't letting the pH get below 6 so you see it bouncing at that number. I then turn off CO2 overnight and it starts again the next morning. The pH rises to over 7.4.

If I understand your statement, I am starting CO2 the next day into water that already has plenty of CO2 and the system pushes another 1.3 degrees to pH 6. This would result in CO2 concentrations much more than 30 ppm even though pH is exactly the same every day. Correct?

If so, I guess it is working for me because I haven't gassed my fish but it does show that things aren't always what you think they are.

Yes. If you have a sump system, the off gassing is much stronger, so your residual levels maybe lower (2-4ppm?) rather than the 5-8ppm there abouts for non sump tanks. The range is quite wide because off-gassing depends on surface area, temperature, flow patterns/speed in a particular tank, indoor CO2 concentrations etc.

"Do you still have the oxyguard? I'd love to see a test in more alkaline water. What about in 0dKH?"

The oxyguard works regardless of the water's alkalinity/acidity, that's why it costs so much. It doesn't interact with water ions at all. It directly measures dissolved CO2 gas through a semi-permeable membrane.

 

[Naturescapes_Rocco]

The oxyguard works regardless of the water's alkalinity/acidity, that's why it costs so much. It doesn't interact with water ions at all. It directly measures dissolved CO2 gas through a semi-permeable membrane.

Sorry, I meant using the oxyguard as a standard to test the Hanna CO2 kit against. Meaning, I'd love to see a more thorough and in-depth testing use/comparison of the Hanna CO2 kit vs a $3,000 CO2 sensor! Other than those 1-2 sentences in your article, there isn't much out there on the internet about these tests or real-world examples using them. I know they (the CO2 test kits) work because I use them, I just want to know what their limitations are against all tank conditions: RO water tests, 0KH tests, high KH tests, basic water, acidic water, etc.

Before I go recommending these test kits to every beginner I want to know their limitations with a scientific basis, and you are the only person I know who has one of these oxyguard machines lol
The hobby would be forever grateful for your findings!

 

[Yugang]

Sorry, I meant using the oxyguard as a standard to test the Hanna CO2 kit against. Meaning, I'd love to see a more thorough and in-depth testing use/comparison of the Hanna CO2 kit vs a $3,000 CO2 sensor! Other than those 1-2 sentences in your article, there isn't much out there on the internet about these tests or real-world examples using them. I know they (the CO2 test kits) work because I use them, I just want to know what their limitations are against all tank conditions: RO water tests, 0KH tests, high KH tests, basic water, acidic water, etc.

Before I go recommending these test kits to every beginner I want to know their limitations with a scientific basis, and you are the only person I know who has one of these oxyguard machines lol
The hobby would be forever grateful for your findings!

The accuracy of the measurement will be dependent on amongst others:

• Calibration
• CO2 ppm - lower concentration will be more challenging, not useful at atmospheric levels
• Flow of water around the probe - Will be very slow at low flow
• Measurement time.

Abstract​

The OxyGuard CO2 Analyzer is a novel meter that can directly measure aqueous CO2 gas pressure using a water-resistant gas-permeable membrane and infra-red absorption cell. The pCO2 is converted to a concentration via a solubility factor determined from the calibration procedure and a thermistor. We undertook to independently validate the precision and utility of this meter. Water flow over the probe membrane was a key determinant of the reaction time. At water velocities of ≥30 cm s−1 the time to 99% span was 6–7 min, while at 0 cm s−1 it was 55–60 min. Temperature and CO2(aq)concentration did not appreciably affect reaction time. The meter had a precision of ±0.5 mg L−1 CO2(aq), and high linearity (correlation 0.99–1.01) above 1 mg L−1 in both freshwater and seawater. The standard meter will not be useful for measuring low concentrations such as atmospheric CO2 levels, but will be useful in situations where accurate pH and carbonate alkalinity determinations are difficult to obtain, such as saline waters and waters of high organic loadings.

 

[Art]

Yes. If you have a sump system, the off gassing is much stronger

I don’t have a sump but my tank is an AIO. I suppose the built in filtration functions similar to a sump allowing for more degassing.

 

[Yugang]

Yes. If you have a sump system, the off gassing is much stronger, so your residual levels maybe lower (2-4ppm?) rather than the 5-8ppm there abouts for non sump tanks.

Assuming water is not stagnant, the rate of off gassing is proportional to the surface area and roughly proportional to CO2 ppm difference to equilibrium.

A sump will add to the surface area, so indeed you lose more CO2 at any given CO2 ppm in the system (tank plus sump). But as the total volume of water, and hence the CO2 in it, is now including the volume of water in the sump, it does not make much difference on the ppm measured in the morning. If the sump is very shallow as compared to the tank, then yes, but generally the sump is just an extension of your aquarium from a surface area as well as volume perspective.

I don’t have a sump but my tank is an AIO. I suppose the built in filtration functions similar to a sump allowing for more degassing.

Only is it adds surface area to the system where gas can escape from water to ambient air.

 

[SingAlongWithTsing]

I don’t have a sump but my tank is an AIO. I suppose the built in filtration functions similar to a sump allowing for more degassing.

just slap a lid on the aio section if you dont already have one.

 

[Art]

No need for a lid. I actually want more aeration and degassing.

My tank has two weirs at opposite ends of the back wall. The water flows through them, through filter wool and down the first compartments. It then flows up, through biomedia in the second compartment and overflows into the center compartment. From there it is pumped back into the aquarium and past the CO2 inline diffuser.

This must be enough additional surface area to provide enough degassing so that I'm starting as close to zero ppm CO2 each morning. If it wasn't, my pH would not be showing the way it does.