# CO2 test kits accuracy



## kekon (Aug 1, 2005)

What is more accurate ? Measuring CO2 by means of test kit or basing on KH and pH ? I have Tetra's CO2 kit but it gives me quite high readings. For example, at KH = 3 and pH = 6.7 CO2 conncentration is 17.9 ppm but the test kit shows over 40 ppm. There isn't any acid source in my tank lowering pH (besides CO2 itself). The test kit is not out of date either.
I wrote about it to Tetra company and they were very worried and astonished about it and also, they asked me to send them my test kit and about 1L of water from my tank. I didn't do that for i was too lazy... I know we cannot rely upon test kits but i wonder if you have the same issues with CO2 test kits (however, the test kit sometimes shows the same value as calculated from KH and pH)


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## LITTLE_FISH (Sep 7, 2005)

I find it interesting that the "Tetra company" is very concerned and asks for an sample before eliminating all other variables first. For example, how did you get the readings for KH and ph? I would assume you used test kits, right? Who says that these tests are reliable? For example, what if the real KH is more like 4, and the real ph is more like 6.6 - in this case, your CO2 would be already around 30. Now, given that the CO2 test kit is a little off, somewhere in that range could be your CO2.

I started out believing strongly in the readings of test kits, but by now I am using them more to evaluate changes, like having the ph lower (lighter color) than I had at some other point.

Ingo


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## kekon (Aug 1, 2005)

LITTLE_FISH said:


> I started out believing strongly in the readings of test kits, but by now I am using them more to evaluate changes, like having the ph lower (lighter color) than I had at some other point.
> Ingo


Yes, i agree that we should be detecting only changes using test kits rather than trying to read accurate values. As far as NO3 and PO4 test kits go, i calibrated them according to "reference solution" of known NO3 and PO4.


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## Bert H (Mar 2, 2004)

> What is more accurate ? Measuring CO2 by means of test kit or basing on KH and pH ?


IMHO, I would trust the kh/pH more, assuming your kits are accurate. Best is to take a sample of your tank water and let it degas 24hours. Measure the pH and compare it to the pH of your tank. A 1.0 pH change (lower in the tank) equals approx 30ppm CO2.


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## hoppycalif (Apr 7, 2005)

Bert H said:


> IMHO, I would trust the kh/pH more, assuming your kits are accurate. Best is to take a sample of your tank water and let it degas 24hours. Measure the pH and compare it to the pH of your tank. A 1.0 pH change (lower in the tank) equals approx 30ppm CO2.


The 1.0 pH change may indicate 30 ppm and it may not. That test depends upon knowing that the degassed sample has about 3 ppm of CO2 in it, but some testing has shown the pH of the degassing sample to continue to rise for even 48 hours, until it shows a .5 ppm level of CO2 in the water. Like the KH/pH test, this may be best used to see if your CO2 level has changed from where you had found it to be high enough to inhibit algae, but low enough not to harm the fish.

An article I found on the internet, and I don't now have the link, said that measuring CO2 in water is very, very hard to do accurately, because CO2 in water is both chemically combined with the solution, and just dissolved in the solution. You can buy CO2 meters for water, but they are lab equipment costing hundreds of dollars or even more. I bought my pH meter thinking I would finally be able to know how much CO2 I have, but now I am just watching the fish and letting them tell me that.


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