Anyone ever use a CO2 test kit?


Junior Poster
Oct 1, 2007
Red Sea Test

I haven't used the Hach test, but have used the Red Sea test. Before replying, I read the Hach manual. Both tests are effectively pH based. They use a titration agent that reacts (changes color) with pH and sodium hydroxide (a base) to neutralize acids (carbonic acid from CO2) in water. They are only as accurate as a drop checker with tank water in it, i.e., not very.


Guru Class Expert
Jun 8, 2007
If they work by neutralizing all acids in the water, I guess it wouldn't really be any more accurate than using the co2 formula either, i.e. additional acids such as tannins will throw it off the same way it will throw out the formula. I'd sooner save my money.


Lifetime Charter Member
Lifetime Member
Jan 24, 2005
I wonder what Tom uses in the field for CO2 tests? Not a drop checker, I bet.


Tom Barr

Staff member
Jan 23, 2005
Nope, I do not use these.
They are non specific for alkalinity, so it's not all carbonate hardness in the alkalinity of the sample, nor does it account for peat/tannins/humic acids etc.

They have the same issues as a pH measure.

Now you can do a two part reference approach.
Use a known KH and an unknown KH/inteference sample that are both in equilibrium with air after 48 hours.

Measure them, then subtract the difference.

Say the known KH has a pH of 7.5
The unknown has a pH of 7.2

You use the pH/KH chart, but remove/subtract 0.3 pH units for the actual measurement for CO2.

I'm not sure why more folks have not done this, I did this method to account for peat and other issues some 10 years ago and posted it on the APD.

It's not perfect, but drop checkers are not either.

The other issues: KH variability through time, peat and other acids influence through time all change, as does the tap's KH.

Folks often just measure pH often...........and rarely test the KH in the tap water, they assume it's stable, generally it moves around and can a great deal depending on the tap water supply.

While the subtraction method works fairly well, there may not be a linear relationship. So while at a the pH's of 7.5 and 7.2 work okay, when you add CO2 and drop the pH down at 6.4 and subtract 0.3 from there, it may not be the same.

To address this, you can take the known KH reference sample and add CO2 as well as the sample from the tank water.

Lower the pH using CO2 for both samples and see if the difference is the same.
I think it is for most part.

This is a good method for folks using pH meters.

Tom Barr


Guru Class Expert
Jun 8, 2007
I mentioned this on another post but wanted to run it by you (and anyone else) here to see if this would be likely to also work.

If you had distilled water with a known KH and let it get to equilibrium along with a sample from the tank, then you could measure the co2 of the known KH using the formula and theoretically it would be identical to the co2 of the tank water after equilibrium. Knowing the ending point of the co2 and testing the pH both before and after equlibrium, wouldn't it be easy to figure out exactly how much co2 you had in the tank to start with? True, there would be a delay factor, but if you don't change the water chemistry of the tank between taking the original sample and testing the pH later on, you should be able to measure the tank water directly and use the offgassed pH of the tank water and the co2 level of the known KH water to calculate the tank's current co2 level by it's current pH.

For instance:

Known KH water after getting to equlibrium: pH 7.2, KH 20ppm
plug into the co2 formula and you get a co2 ppm of 2.11

After offgassing tank water sample: pH 7.0, KH of 40ppm (but the KH is irrelevant)
Now assuming that this water at this point has a co2 ppm of 2.11 also...

You go back and test the pH of the tank again (assuming you haven't messed with the water chemistry) and you get a pH of 6.2. If you started at 6.2, and ended at 7.0 with 2.11 ppm of co2....a change of .8 pH represents 8x more (or less) co2....

So 8 x 2.11ppm = 16.88ppm is what's in the tank.

Measuring KH on the tank doesn't matter, it's the pH that tells you what you want to know. Assuming that there is a possibility of .1 error, you could realistically have between 7 - 9 x the co2 level, or from about 14 - 19ppm.

The only thing I don't know right now is how the pH/co2 relationship changes when you get beyond a 1.0 change in pH which is equivalent to 10x the co2. I know it's not exponential like pH (which is good because it affords a larger margin of error).


Lifetime Charter Member
Lifetime Member
Jan 24, 2005
Sacramento, CA
If a sample of water has a pH of A right after it is taken from the tank, and that pH increases to B after you wait long enough for it to be at equilibrium with the atmosphere ( assuming you didn't allow a significant amount of the sample to evaporate), and you have determined that the ppm of CO2 at equilibrium with the air is C, the ppm of CO2 in the sample when it had a pH of A is D:
D = C x 10 exp(B - A)
ppm of CO2 in a sample equals known ppm of water at equilibrium with the atmosphere times ten raised to the power of the pH at equilibrium with the air minus the pH of the sample when you first removed it from the tank.