# New CO2 Question



## rti1000 (Jun 6, 2005)

I'm new to planted tanks and i had a question about using CO2 to control Ph. I believe i understand the relationship between Ph, KH, and CO2. The Ph in my tank without co2 stays between 6.8 - 7 consistently but in order to inject co2 i need to raise my KH levels (which are near 0) and i would like a target Ph of 6.4 which means i would need to raise my KH to 2.5 if i want to keep 30ppm co2 in my tank. correct?

This leads me to my next question. If you have a co2 system with a Ph controller why not leave it on all the time and let the controller dictate how much co2 to inject based on the plants usage or lack thereof instead of turning the system on and off with the lights at night? If i had a KH of 2.5 and set a controller to inject co2 to a Ph of 6.4 and i shut the system off at night my Ph would raise to 7.0 - 7.2 if the co2 levels dropped to 8 - 5 ppm right? Wouldn't that cause an unhealthy environment for my fish, namely Discus?


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## JeffB (Mar 24, 2005)

You raise the KH so that adding C02 will not give a drastic ph swing. If you leave the KH at 1 with the C02 at 30 ppm you will have a ph of 6.0. 

The higher the KH the less the PH will move.

I cant answer your second question though direct experience, but I do know that the controllers are rated +/- .2 accuracy so the lower the KH the greater the swing even with a controller.


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## Laith (Sep 4, 2004)

Welcome to APC!

Yes, a KH of 2.5 and a pH of 6.4 gives you a CO2 concentration of 30mg/l. Be aware that a KH of 2.5 is about as low as you want to get in order to avoid any risk of a pH crash.

I have a pH controller and I leave it on all the time. For me turning the pH controller off sort of defeats the purpose of the thing...

Some people who don't use a controller do turn off the CO2 when the lights are off which makes sense as there's nothing to stop the CO2 levels continuously climbing when the plants aren't using it.

A pH controller turns off the CO2 at the set pH level so there is none of that risk.


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