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CO2/KH Relationship and pH Monitor/Controller Accuracy

1798 Views 8 Replies 4 Participants Last post by  Edward
Consider the range of CO2 levels (ppm) at these KH/ph combinations (based on Chuck Gadd's calculator :

When KH = 4 and
pH = 6.6 then CO2 = 38
pH = 6.8 then CO2 = 24
ph = 7.0 then CO2 = 15


When KH = 5 and
pH = 6.6 then CO2 = 30
pH = 6.8 then CO2 = 19
ph = 7.0 then CO2 = 12

Now, assume you're using the Milwaukee SMS122 pH controller with a stated accuracy of +/- 0.2 pH units.

I take that to mean a reading/set point of 6.8 could actually be as low as 6.6 or as high as 7.0. Is that a correct assumption?

If so, you can see the wide range of resulting CO2 levels at the given KH. With a KH of 4, you could have a CO2 levels ranging from 15 (maybe not high enough for plants) to 38 (perhaps too high for fish?)

Am I missing something or is that the way it would work?

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Hi trc_pdx,

you didn't read the table correctly, but that's not the issue here.
The second part, about the regulator accuracy is correct. This is why we should keep the CO2 30 - 40 ppm. I use 60 ppm with Cardinals, no problem.

Thanks Edward. Yes, I have the 4.0 and 5.0 KH values swapped. I wonder if others have found the frequently cited warnings about CO2 levels above 30 being harmful (to fish) to be erroneous.

Hi trc_pdx,

had CO2 at 70 - 90 ppm with Tetras. Nothing was wrong, they were ok.

had CO2 at 70 - 90 ppm with Tetras. Nothing was wrong, they were ok
Wow. The Gadd calculator warns you off at 25ppm.

This probably belongs in the Equipment forum but I wonder if other users of the Milwaukee controller have found it more or less accurate than the specified +/- 0.2 accuracy.

Edward, you had 70-90ppm without any surface movement what-so-ever? I'm guessing you don't have shrimp either..?

I don't have any shrimps and I always have an air stone running with fish.

Edward you use an airstone in your planted tanks? Doesnt that drive your CO2 off? I have always been told not to use an airstone with pressurized CO2.
Hi chiahead,

All my tanks with fish have air stones, regulated to low flow.
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