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Discussion Starter · #1 ·
So with pressurized CO2 my pH is around 6.4 and my KH is either 3 or 4 (I couldn't tell). The only thing that has changed recently was me doing a 50% water change of RO water instead of tap.

There are no signs of oxygen problems with my fish though so I can't imagine there is too much CO2.
 

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Discussion Starter · #3 ·
In my tank with CO2 KH is around 2 and pH sometimes gets as low as 6.2, everything is fine though. So don't worry, just be careful of sudden big changes of pH.
Ok, I was thinking it was because of the big water change because my tap pH is pretty high.

According the kH/pH chart with my kH being around 3-4 and my pH being 6.4ish, it would put my CO2 in the danger zone but there are no signs of stress from my fish.
 

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"pH way too low" for which species? it's too high for many.
 

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RO water has very low mineral content, this relates to a low alkalinity, which makes pH very difficult to stabilize. When you then inject CO2 rather than it creating carbonates you are creating carbonic acid which is driving your pH down.
 

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... I have cory cats
There are many species of Corydoras...
My pH drops to 5.4 or so during the day and my Cory cats are happy as can be. Don't worry about them, they'll be fine...
RO water has very low mineral content, this relates to a low alkalinity, which makes pH very difficult to stabilize...
At least 2 species of Corydoras thrive in pH levels as low as 4.0 Corydoras melini ? False Bandit Cory ? Seriously Fish Corydoras adolfoi ? Adolfo?s Cory ? Seriously Fish

RO water, rainwater, distilled water, has very low mineral content, this relates to a low pH, it's soft it is acidic and it is very low in TDS (Total Dissolved Solids) - which means it contains a higher ratio of real, pure water. (Less TDS = more water, as water is a liquid not a solid.)

This as opposed to high mineral content, high pH, hard, alkaline, and high in TDS - which means it contains a lower ratio of real, pure water. (More TDS = less water.)

Species which are known to thrive in water of the former description are also much more adapted to fluctuations in pH, as wind, rain, drought, bioload, etc, all cause wider pH fluctuations in "blackwater" regions than in regions of alkaline waterways.

Therefore that it "makes pH very difficult to stabilize" is moot, as acidic water fish have been in it since the dinosaurs.
 
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RO water has very low mineral content, this relates to a low alkalinity, which makes pH very difficult to stabilize. When you then inject CO2 rather than it creating carbonates you are creating carbonic acid which is driving your pH down.
If you add 30 ppm of CO2 to any water it will drop the pH by about 1.0, whether the pH starts at 7.6 or 6.6. In other words you can't buffer the water against pH changes caused by CO2. But, you can buffer it against pH changes caused by other acidic substances, such as tannic acid. Low alkalinity just means the pH will be low vs what it would be with high alkalinity.
 
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