# CO2 relationship to KH



## Miccoh Mendoza (12 Jun 2018)

Hi everyone,

I am setting up a new planter tank and planning to use CO2. After getting a KH kit and testing my current fish tank, my KH is about 24 and Ph is 8.2-8.4. Looking at the chart for CO2 it is in the low side. I was just wonder does CO2 injection lower KH? And should I increase CO2 to get it down to idea level for plant?


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## ian_m (12 Jun 2018)

Ignore the chart, you are heading down the path of poor plants and maybe even fish death.

Either aim for a green drop checker, which works regardless of tanks water kH or a pH drop of 1 unit from CO2 start to lights on, again completely independent of tank water kH. Job done, no unreliable test kits required.


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## dw1305 (12 Jun 2018)

Hi all, 





Miccoh Mendoza said:


> Hi everyone,
> 
> I am setting up a new planter tank and planning to use CO2. After getting a KH kit and testing my current fish tank, my KH is about 24 and Ph is 8.2-8.4. Looking at the chart for CO2 it is in the low side. I was just wonder does CO2 injection lower KH? And should I increase CO2 to get it down to idea level for plant?


The CO2 ~ carbonate ~ pH equilibrium is a bit strange. The total amount of inorganic carbon doesn't change, it just changes form (in the graph below).




When you add CO2 you change the pH equilibrium values, but you don't change the alkalinity. 





ian_m said:


> Either aim for a green drop checker, which works regardless of tanks water kH or a pH drop of 1 unit from CO2 start to lights on, again completely independent of tank water kH. Job done, no unreliable test kits required


<"I'm not a CO2 user">, but I agree with @ian_m you really need a <"drop checker with 4dKH solution and bromothymol blue pH indicator">.

cheers Darrel


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## ceg4048 (22 Jun 2018)

Miccoh Mendoza said:


> I am setting up a new planter tank and planning to use CO2. After getting a KH kit and testing my current fish tank, my KH is about 24 and Ph is 8.2-8.4. Looking at the chart for CO2 it is in the low side. I was just wonder does CO2 injection lower KH? And should I increase CO2 to get it down to idea level for plant?


Just aim for a pH drop of anywhere from 0.5 to 1.0 depending on the amount of light intensity you have. Start the injection early enough so that the drop is achieved by the time the lights turn on. As mentioned by Ian and Darrel, use a dropchecker to give you a visual guide to the CO2 level.

Cheers,


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## REDSTEVEO (22 Jun 2018)

Did I read that correctly, KH is 24, and PH is 8.2 to 8.4??? Regardless of CO2 that sounds like really hard water, hard enough to keep lake Malawi cichlids or even marine fish.

What is driving the PH and the KH so high. If you've got fish in there good luck with them, 

If I've misread the figures fair enough.


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