PH and calculating CO2 concentration - The Planted Tank Forum
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post #1 of 8 (permalink) Old 07-21-2016, 05:12 PM Thread Starter
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PH and calculating CO2 concentration

I have a 75 gallon tank with an FX5 filter, with a cerges reactor, inline heater and UV Sterilizer. I have recently hooked up an Apex Classic Controller for the tank and have been watching my CO2 levels through the day. I have seen that 30ppm of CO2 is the amount you want to have in your tank for optimal plant growth. I have 1dKh in my water and when the CO2 kicks on the ph is 6.9 and then once the middle of the lighting period (8 hours high light BMLED) it drops to 6.25 which is only a .65 drop in ph. I have read on the forum that you should achieve a 1 degree ph drop. Is this 1 degree ph drop based off of the night peak ph? or based off of a completely off gassed water reading? Does the 1 degree drop matter as long as you have 30ppm during lights on time?

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post #2 of 8 (permalink) Old 07-21-2016, 05:36 PM
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Complete off gas. Several things effect your pH to reading as it relates to co2. The one degree drop is just a good starting point which will hopefully approximate 30 ppm. I generally start there and then raise it until the fish look stressed then drop it a tad.

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post #3 of 8 (permalink) Old 07-21-2016, 05:38 PM
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Please ignore the recommended ranges for good/Bad CO2 ppm, they vary by tank and lighting and other factors, a good range to target initially is 30-40 ppm.
Then adjust slowly watching plant health and vigor.
http://www.barrreport.com/forum/barr...o2-ph-kh-table

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post #4 of 8 (permalink) Old 07-21-2016, 05:41 PM
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And yes, you want the co2 concentration consistent during the lighting period.

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post #5 of 8 (permalink) Old 07-21-2016, 06:21 PM Thread Starter
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Thanks for the input, I will have to figure out how to get my ppm a bit higher, I don't seem to be quite there yet.
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post #6 of 8 (permalink) Old 07-23-2016, 06:58 PM
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The optimum concentration of CO2 in the water depends on how much light you are using, and whether the plants you have are fast growers or not. When use of CO2 was still a novelty we needed to know what the ballpark number was for "good" CO2, so people, using the crude methods for calculating CO2 concentration, found that fish generally could live with 30 ppm, but might not live with more. So, that 30 ppm number was based on how the fish tolerated it, not what the plants needed.

If your lighting is around 30-40 PAR the plants will grow well with much less than 30 ppm of CO2, but if it is around 80-100 PAR, the plants will be stressed by having too little CO2, and algae will tend to take over the tank.

It isn't possible to accurately measure how much CO2 is in the water without spending a lot of money for an accurate CO2 probe. If you can be sure that your water has 3 ppm of CO2 in it from the atmospheric CO2, a drop in pH of 1.0 that results from adding more CO2 means you have 30 ppm of CO2. (That is 10 to the 1.0 power times the original pH.) But, atmospheric CO2 doesn't result in exactly 3 ppm of CO2 in the water. 3 ppm is just a reasonable approximation.

Also, using the pH/KH vs ppm of CO2 tables is not accurate unless the water you are testing contains nothing that affects KH or pH except carbonates and CO2. Typical aquarium water does contain other substances that affect both the KH and pH. Even if that isn't true for your water, it is very hard to accurately measure pH, unless you have a well calibrated pH probe/meter, and the calculated ppm of CO2 changes drastically with small errors in measured pH.

We just have to live with the crude nature of our CO2 ppm measurements, and accept that we don't know with any accuracy how much CO2 we have.

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post #7 of 8 (permalink) Old 07-24-2016, 02:29 AM
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For what it is worth, here is the math behind the chart.


Not much trust in the chart.
More faith in degassing a sample and assuming 1.0pH = 30ppm CO2.


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Growing is not that difficult.
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post #8 of 8 (permalink) Old 07-24-2016, 02:34 AM
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Good advice above, good read here

CO2/pH/KH table - Aquarium Plants - Barr Report
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