I think it might be a good idea to have a place dedicated to this issue where information is in one place and easier to search. Also, I welcome everyone with knowledge in this field to help us.
Further details and theory.
First we look at Henry's law and the solubility of gases.
Gases will dissolve in liquids to an extent that is determined by the equilibrium between the undissolved gas and the gas that has dissolved in the liquid. The equilibrium constant for that equilibrium is:

k = px / Cx

Where :
k = the equilibrium constant for the solvation process
px = partial pressure of gas x in equilibrium with a solution containing some of the gas
Cx = the concentration of gas x in the liquid solution

The form of the equilibrium constant shows that the concentration of a solute gas in a solution is directly proportional to the partial pressure of that gas above the solution. Meaning that doubling partial pressure of CO2 will double the concentration of CO2 in the liquid solution.



To get CO2 partial pressure from CO2 concentration we look at Dalton's law of partial pressure.
The pressure exerted by an individual gas in a mixture is known as its partial pressure. Therefor the contribution of gas to the total pressure is its partial pressure. Since the gas molecules in an ideal gas behave independently of other gases in the mixture, the partial pressure of CO2 is the same pressure as if there were no other gases in the container. Therefore, if we want to know the partial pressure of CO2 gas in the mixture, we can completely ignore the other gases. The pressures are independent of each other.

For the result we can use Calculator of Dalton’s Partial Pressure.
Enter moles to 200 and 400, while set Temperature to 273 K and Volume to 4539.45 L. The two results reveal that doubling atmospheric concentration of CO2 will double the partial pressure of CO2, see 100 and 200 kPa pressure results.





When doubling atmospheric concentration of CO2 doubles the partial pressure of CO2 then it also doubles the concentration of CO2 in the aquarium water column. Atmospheric variations of CO2 levels will significantly be affecting the final water column CO2 levels as seen above. These differences increase with higher pH drop due to the logarithmic scale of pH readings.

References:
https://en.wikipedia.org/wiki/Partia...ility_of_gases
https://www.khanacademy.org/science/...rtial-pressure
https://wpcalc.com/en/daltons-law

@ Positron
Thank you for the info.
Why do you think “The actual balance between the two is based on CO2 partial pressures combined with everything else in the air (N, O, H ect).” , when the Dalton’s law says gas behave independently and other gasses can be completely ignored. The pressures are independent of each other.

Thanks

Once I get into CO2, this will come in quite handy, thanks.

For those interested in CO2 levels and bioavailability this chart explains one of possible reasons why some plants grow better in lower KH.




When atmospheric or injected CO2 gas enters water it dissolves and some react with water to form carbonic acid H2CO3. This carbonic acid H2CO3 will dissociate to form bicarbonate HCO3. And then the bicarbonate HCO3 will dissociate further to carbonate CO3. However, these reactions are pH dependent. Water pH determines what form carbon will have. Since some plants can use carbon in bicarbonate HCO3 and carbonate CO3 forms most prefer dissolved CO2 and carbonic acid H2CO3. When we look at the chart, we can see the red line showing dissolved CO2 and carbonic acid H2CO3 forms rapidly decreasing 50% in pH of 6. The decrease of these preferred carbon forms is 87.5% between pH of 5.5 and 7.0. In other words, the same CO2 supply can have only 12.5% available carbon in the preferred form by just having the wrong pH. And this is why KH is so important because KH is affecting pH which dictates what form carbon will be in.

Also interesting is that 1 pH drop from pH of 7.5 to 6.5 has 3 x less carbon in the preferred form than the same pH drop from pH of 6.5 to 5.5.

One slight clarification: For water in equilibrium with some partial pressure of carbon dioxide gas, the amount of dissolved carbon dioxide is determined solely by the gaseous carbon dioxide partial pressure. pH doesn't actually affect it. However, at higher pH, you will have a lot more carbonic acid and bicarbonate, and if your plants can use these, they're better off. In other words, pH changes carbonic acid and bicarbonate relative to a fixed level of dissolved CO2.



Of course, if you're injecting CO2, you're not in equilibrium. You may not be in equilibrium even in a non-injected tank, if plants or fish are active enough. And this is a good thing: If your tank was actually in equilibrium with pure CO2 at atmospheric pressure, which is the ultimate result of perfectly efficient injection of massive quantities of CO2, your pH would be floored and your fish would be floating belly-up on top of your tank.

Thank you for this! I'll definitely try to digest it more once I'm home.

Good base-line topic! I'm going to have to digest this over some time. To generalize, without going into the math, I just want to add a quote from Dr. Morin that I've kept (can't recall where I got it):

In 'normal' tanks, this is shown by many to create CO2 levels in the 3-4ppm area.

Sure wish there was a good way to easily (cheaply) measure CO2 uptake in our individual tanks. Having a known KH and monitoring pH probably wouldn't do it because you don't know how fast CO2 is gassing off in a given tank.

Interesting stuff Edward.

For some reason the chart is not showing up??

@Dianna Walstad published the concentration of atmospheric CO2 equilibrium with water, based on reference by Wetzel RG. 1983. Limnology (Second Ed.). Saunders College Publishing (Philadelphia, PA), p. 202 as 0.5 mg/l, (0.5 ppm). Ecology of the Planted Aquarium, page 100, “Water in equilibrium with air contains 0.5 mg/l CO2”.

The widely used pH drop idea to control water concentration of CO2 is based on equilibrium of 3 ppm. Recommended 1 pH drop represents 3 x 10 = 30 ppm CO2. However, if Dianna Walstad reference is correct then people actually have 0.5 x 10 = 5 ppm CO2. The reason why 0.5 ppm is actually possible is because people often have 1.6 pH drop and healthy fish at the same time. The pH drop of 1.6 is 20 ppm CO2, instead of 120 ppm CO2 at 3 ppm degassed.

Anyone have any information on where the original degassed 3 ppm CO2 came from?

Oof. I'm so let down now.

*Runs over to the tank and starts cranking the gas*

That's a big if.

Is that number referenced anywhere else?

And in the end, it's of little consequence. While it would be interesting to know the absolute value, controlling relative value is what's important.

That being said, do you know if anyone has used high end test equipment to get the bottom of this? I would be interested to see those results.

@Edward just continues to ruin our dreams!

Old data from '83' must be wrong.

I have several old wives that I know, if you'd like to talk to them.

More and more, i'm thinking that a well-tuned dc is, after all, the best indicator.

I can tell you this.

My degassed pH/KH readings indicate a CO2 concentration between 3.0 and 4.0.......like clock work.

With my tank disaster, my KH has been all over the place for over a month. I can test KH and accurately predict degassed pH every time.

And some time back this topic came up and I calculated a bunch of members degassed CO2 levels based on their posted tank spreadsheets........and yep somewhere around 3.0 to 4.0 every time.

Now whether our KH readings are totally accurate is another question.

And in no way is this proof, just an observation. Still would love to see high end test equipment used to delve deeper.

Actually, I’ve been one of us that have also supported the 3-4 ppm equilibrium, which I thought was proven by Henry’s law but, recently, I’ve started to question it based upon some of the stuff that @Edward is showing, as well as what @cl3537 discussed. I’ve looked around and found that the variables and math do seem to support this, but I also still see compelling explanations to expect 3-4 ppm. Frankly, right now, I’m not sure what the real answer is or how someone might verify (calibrate) the math without reading the CO2 directly, which would be pretty expensive and involved. I’m not talking about the CO2 indirect inference-type tests that hobbyists use and are <$100.

However, although a dc won’t tell us precisely where CO2 is, it should be reliable in terms of proximity, according to what I’ve read and knowing how it works with only the one variable involved. This assumes that we truly know the KH of the indicator solution and this decays, as you know, in a few weeks.

I’m not so sure that I can recommend the 1-point drop approach anymore. It would be nice to be able to have a chart that would show how much a 1-point drop equates to x-amount of CO2 based upon the variables that are supposed to influence it, if the simple 1-point drop from any pH level is no longer (or never was) valid. For example; if you are confident that a 1-point drop from your degassed pH is 30 ppm, do you also believe that a further 1-point drop from your fully gassed level would be another 30 ppm?