2 point drop in ph from CO2 - The Planted Tank Forum
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post #1 of 12 (permalink) Old 07-03-2020, 09:48 PM Thread Starter
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2 point drop in ph from CO2

I just measured my ph drop from CO2 and my ph goes from 8 to 6. I used a calibrated CO2 pen.

The fish seem fine. Has anyone ever seen this before?

According to the PH/KH chart I have 1.1 ppm co2 degassed and 270 ppm with co2.


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post #2 of 12 (permalink) Old 07-03-2020, 10:15 PM
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First off those pH/kh charts use assumptions that are oftentimes incorrect. So it's best to not pay too much attention to those charts.

Now as for your 2.0 pH drop, it might be possible to get to that level if your atmospheric co2 is fairly low. Again, those charts don't paint an accurate picture of what your starting concentration is at. The starting concentration is also dependant on atmospheric pressure. Less pressure, less starting co2.

And since pH is a logarithmic scale a drop of 1.0 equals a 10x increase in co2.

So if you are in an area with low atmospheric pressure and, amazingly enough in this day and age of global warming, an area of low co2 concentrations you can conceivably get a 2.0 pH drop.

Most get somewhere around 1.5 with no outward ill-effects.

That or your pH pen is inaccurate at low pH readings. :-)

Fyi... I have a pH monitor that records 0.3 lower than it should when in the aquarium yet I can calibrate to 7.0 and 4.0 solutions successfully.

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post #3 of 12 (permalink) Old 07-03-2020, 10:28 PM Thread Starter
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I am at a high altitude so that might explain it.


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post #4 of 12 (permalink) Old 07-03-2020, 11:47 PM
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Quote:
Originally Posted by pauld738 View Post

That or your pH pen is inaccurate at low pH readings. :-)
This is my first guess. What's your KH? (I realize I could probably study the chart and figure it out on my own)

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post #5 of 12 (permalink) Old 07-04-2020, 12:52 AM
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What's your KH?

CO2 over 30ppm is dangerous for living creatures so your CO2 measurement of 270ppm is totally incorrect.

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post #6 of 12 (permalink) Old 07-04-2020, 01:04 AM
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Not that it matters that much in this situation...

But I'm betting that the initial pH reading is close to correct which would put Kh somewhere around 6dKh. Give or take a degree. :-)



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post #7 of 12 (permalink) Old 07-04-2020, 05:43 AM
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Quote:
Originally Posted by pauld738 View Post
First off those pH/kh charts use assumptions that are oftentimes incorrect. So it's best to not pay too much attention to those charts.

Now as for your 2.0 pH drop, it might be possible to get to that level if your atmospheric co2 is fairly low. Again, those charts don't paint an accurate picture of what your starting concentration is at. The starting concentration is also dependant on atmospheric pressure. Less pressure, less starting co2.

And since pH is a logarithmic scale a drop of 1.0 equals a 10x increase in co2.

So if you are in an area with low atmospheric pressure and, amazingly enough in this day and age of global warming, an area of low co2 concentrations you can conceivably get a 2.0 pH drop.

Most get somewhere around 1.5 with no outward ill-effects.

That or your pH pen is inaccurate at low pH readings. :-)

Fyi... I have a pH monitor that records 0.3 lower than it should when in the aquarium yet I can calibrate to 7.0 and 4.0 solutions successfully.

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post #8 of 12 (permalink) Old 07-06-2020, 12:20 AM
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That or your pH pen is inaccurate at low pH readings. :-)

Double-check your ph pen with a reagent test. I have an inexpensive ph pen that I stopped using due to inaccuracy. If you're not already doing so, keep a co2 checker installed in your tank, even the inexpensive Fluval one will do. This way you have an easy visual if your co2 is at a dangerous level.

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post #9 of 12 (permalink) Old 07-06-2020, 05:45 PM
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another factor: how do you obtain your baseline pH? when and where matter.
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post #10 of 12 (permalink) Old 07-08-2020, 10:38 PM
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Can you link your kh/ph table. My female bn died and my ph only dropped by 1 which should have been fine with a kh of 3. Anyway doesn't help with your issue but just sort of my way saying be careful with too much co2 if the tank has fishes.
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post #11 of 12 (permalink) Old 07-08-2020, 11:54 PM
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This popped up in the recent discussions and I made similar comments on CO2 measurements so I wanted to add my two cents.

The above comments I think are all valid; concerns about off-gassing in high altitude/low air pressure is probably true, pH pen inaccuracies, and issues with the KH/pH chart. My comment involves the latter in that the chart assumes KH or carbonates is your only buffer in the tank. Now, that may be true as we don't know what else is in the tank, but there are plenty of other factors affecting pH including rocks, active substrates, chemical filtration media, and phosphate levels (though this last one is not likely an issue in most tanks). All that to say, take pH drops as one estimate of CO2 levels and I'd echo the comment to use a drop checker as another. I would guess you have very low KH and perhaps no other means of buffering the tank pH, like with an active substrate for example.

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post #12 of 12 (permalink) Old 07-09-2020, 12:53 AM
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pH is one part and KH is the other part. Both have to be calibrated and both should be measured with each pH reading (KH can change, even daily, as a function of many things).

You mentioned that you calibrated your pen. Was this to non-expired 4.0 and 7.0 pH calibration solutions? Did you also calibrate your KH test to a known KH level? If dKH is measured incorrectly, even a slight error can give a significantly false CO2 result. Additionally, dKH testing can be less precise than desired. The simple API kit can be greatly improved, in precision, buy using 5x the sample water and dividing the results by 5. I also use a Hanna KH test kit, which provides even greater precision, and you might be surprised at how this shows the API kit to be fairly wanting in precision even when using the higher precision approach to the API kit.
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