Kh / Ph / Co2 Inconsistency

Brian Kimbark

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Jan 2, 2019
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Hi,
I recently purchased a Salifert KH test kit hoping to get a bit of a more accurate reading of my KH than the liquid test strips I had previously been using. I am also using an API liquid test kit to test for PH and a CO2 drop checker with fresh pre- mixed 4kdh solution on my 75 gallon planted tank.
I estimate my PH to be at approximately 6.4 and my KH test shows that my KH is low at 1.05. After putting this information into a calculator, and also referencing the co2 / KH / PH charts my co2 is estimated at 12.5ppm. However, my drop checker has a light green color, which tells me my CO2 should be higher than 12.5ppm.


So what gives? Clearly one of these tests has to be innacurate, right?

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Phishless

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IMO ditch the chart and the drop checker and the API pH kit.
Buy a pH pen with calibration fluid, around $20 on Ebay & usually comes paired with a TDS pen.
Test your tank water de-gassed for 30 minutes or so with an airstone.
Target a 1.0 pH drop after an hour or so into photo period.
Typically this 1.0 drop is estimated at 30ppm CO2 content for our purposes.
 

Brian Kimbark

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Jan 2, 2019
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Also I wouldn't argue over a KH of "1"
I'm trying to get that low myself.
Thanks for the quick reply. I’ll check online for a quality pH pen. So you wouldn’t worry about having a KH of 1? I know lower KH can result in larger PH swings but I have also heard that co2 related pH swings are harmless.
 

Phishless

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1 dKH is enough alkalinity to avoid a crash in pH.
CO2 is a temporary pH alteration that returns to normal.
Carbonic acid is too weak to destroy the alkalinity.
 

Brian Kimbark

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1 dKH is enough alkalinity to avoid a crash in pH.
CO2 is a temporary pH alteration that returns to normal.
Carbonic acid is too weak to destroy the alkalinity.
Thanks a lot. This makes me feel a bit better. Any pH probe or pen that you have experience with and like?
 

Phishless

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If probe comes with @ least two calibration fluid packets you should be good.
They are typically unbranded and could last as long as three years possibly.

Still measure KH occasionally, down the road you may need to add some carbonate or bicarbonate to raise it.
 

Brian Kimbark

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Thanks for the info. I got myself a pH pen and let tank water sit in a container with an air stone for nearly 2 days. I took the air stone out and measured the water at 7.5 pH. Tank water was at 6.5 (about 4 hours into photoperiod).

Today I retested the water that was sitting out overnight, this time without an air stone and it tested at 7.2.

Any idea which reading is likely more reliable when it comes to using it to compare a 1.0 pH drop?
 
Last edited:

tiger15

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Aug 12, 2017
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I bought the same pH and TDS pen a few months ago.

When the pH pen is working, it's great as you can take a lot of data with minimum effort and read to 0.1 precision. But the sensor has short life, and went spooky giving nonsense display after a few months of use. The TDS pen is still working. I trashed the pH pen and returned to use the API liquid indicator. I heard that the pH pen needs to be stored in special liquid buffer to preserve the sensor, but it didn't come with my purchase.

Are you aware that pH pen and liquid indicator can give substantially different readings if there is substantial CO2 mist in the sample. pH pen reads the electrical conductivity signal, and won't pick up gaseous CO2. Liquid indicator reads by chemical reaction, and will pick up both dissolved and gaseous CO2. So if there is CO2 mist, pH pen will underestimate CO2 based on kH-pH chart.
 

echocinco

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Sep 19, 2018
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IMO ditch the chart and the drop checker and the API pH kit.
Buy a pH pen with calibration fluid, around $20 on Ebay & usually comes paired with a TDS pen.
Test your tank water de-gassed for 30 minutes or so with an airstone.
Target a 1.0 pH drop after an hour or so into photo period.
Typically this 1.0 drop is estimated at 30ppm CO2 content for our purposes.
I wouldnt discard your drop checker, but cant argue against getting a pH pen. I like drop checkers even if they arent accurate because they are still consistent. If I know my CO2 levels are good with it at a dark green and I suddenly see it at lime green one day, it serves as a visual clue for me to look into the discrepancy. It becomes a hassle to have to constantly check values, especially if you have multiple tanks.

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