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post #1 of 10 (permalink) Old 04-16-2006, 12:53 AM Thread Starter
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Co2 Charts

I've seen a few Co2 charts in the forums before but they don't seem to match the readings I get. I have the Aquarium Pharm Gh & Kh test kits. My Ph is 6.0 and both my Gh & Kh are 53.7 . Are the Co2 charts specific to a brand of test kit?

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post #2 of 10 (permalink) Old 04-16-2006, 01:31 AM
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if your KH is 3 (53.7), according to the chart i use, you need to raise your ph somewhere in the range of 6.4-7 for optimal Co2...why do you think the charts are incorrect?
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post #3 of 10 (permalink) Old 04-16-2006, 03:01 AM
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You can always just use PH to determine your CO2 amount. Measure PH in the tank with CO2 stabilized, then take a tank water sample, let it sit for several hours (overnight) and measure the PH of that sample. The difference between the two PH readings should be .8 to 1.1 if your CO2 is about 30-40 ppm.

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post #4 of 10 (permalink) Old 04-16-2006, 01:59 PM
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Quote:
Originally Posted by Hoppy
You can always just use PH to determine your CO2 amount. Measure PH in the tank with CO2 stabilized, then take a tank water sample, let it sit for several hours (overnight) and measure the PH of that sample. The difference between the two PH readings should be .8 to 1.1 if your CO2 is about 30-40 ppm.

I don't think that method works. I can lower my PH by 2 and the fish show no signs of stress. My stablized PH is 7.8 and I run my tank at 6.4 or 6.3 during lights on. If I go to 6.6 I get BBA coming on fast. Also just a quick glance at chucks calculator and the sticky in this secton show - for a KH of 3.5 and a starting PH of 7.8 dropping the ph by 1 will add about 10ppm CO2.

Often used CO2 charts and my experience both indicated that a drop of 1 in PH is not a good way to determine if you have 30PPM of CO2.

Rick.
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post #5 of 10 (permalink) Old 04-16-2006, 05:41 PM
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Originally Posted by rrguymon
I don't think that method works. I can lower my PH by 2 and the fish show no signs of stress. My stablized PH is 7.8 and I run my tank at 6.4 or 6.3 during lights on. If I go to 6.6 I get BBA coming on fast. Also just a quick glance at chucks calculator and the sticky in this secton show - for a KH of 3.5 and a starting PH of 7.8 dropping the ph by 1 will add about 10ppm CO2.

Often used CO2 charts and my experience both indicated that a drop of 1 in PH is not a good way to determine if you have 30PPM of CO2.

Rick.
You are misreading the chart - starting at KH =3.5, PH=7.6, dropping to PH = 6.6 at KH = 3.5, gives 26 ppm of CO2. You can verify that the PH drop of 1.0 method works by a simple bit of algebra. Solve the equation for ppm CO2 for KH. Then substitute that for KH in the original equation. Label the PH term in the first equation as PH(deaerated), and the PH term in the final equation as PH(tank water). The final equation shows that a 1.0 drop in PH gives a ppm of CO2 of ten times the deaerated value of CO2, which my experimenting showed, for my conditions, to be about 4. Others think it is 3.

The method using a 1.0 drop in PH does indeed work, and if nothing is amiss in your tank water it will give the same result as the KH/PH table.

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post #6 of 10 (permalink) Old 04-16-2006, 06:21 PM
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Quote:
Originally Posted by Hoppy
You are misreading the chart - starting at KH =3.5, PH=7.6, dropping to PH = 6.6 at KH = 3.5, gives 26 ppm of CO2. You can verify that the PH drop of 1.0 method works by a simple bit of algebra. Solve the equation for ppm CO2 for KH. Then substitute that for KH in the original equation. Label the PH term in the first equation as PH(deaerated), and the PH term in the final equation as PH(tank water). The final equation shows that a 1.0 drop in PH gives a ppm of CO2 of ten times the deaerated value of CO2, which my experimenting showed, for my conditions, to be about 4. Others think it is 3.

The method using a 1.0 drop in PH does indeed work, and if nothing is amiss in your tank water it will give the same result as the KH/PH table.
Why are you picking and choosing off the chart? My example is 7.8 to 6.8 with kh of 3.5 not 7.6 to 6.6. For 7.8 to 6.8 co2 is 17 not anywhere near 30. I think something is wrong with your math.

That being said my water is a bit unusual and the normal ph kh charts do not work well ether. The charts assume that most of the water kh is made up from bi carbonates. That is not the case in my water, Almost half of my KH is made up from something else. KH test kits indicate my kh is 6.5 or 7. However the the bicarb component is only about 3 to 3.5. If I use 3.5 as my kh reference the charts works great. FYI the water utility was able to tell me how much of my KH was bicarb.

Rick

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post #7 of 10 (permalink) Old 04-16-2006, 06:40 PM
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Sorry, Rick, I misread your post. The problem is, at 7.8 PH, the KH will not be 3.5, because the amount of CO2 in the water will be about 3 to 4 ppm, not 1.7 ppm. The method of using 1.0 PH drop to get 30 to 40 ppm of CO2 is based on the deaerated water having about 3 to 4 ppm of CO2 in it, which my testing showed it to have (but, that might not be true for someone at way above sea level). It is a fun experiment to try to establish what the ppm of CO2 is in your degassed tank water - I used RO, carbon filtered water from the grocery store. I added a tiny bit of bicarbonate to get a KH, then diluted that with more RO, carbon filtered water to get the KH down to around 2. I let the sample stand a few hours - I found that 4 hours was enough - measured KH and PH and used the chart to get the ppm of CO2. Several of these tests at varying KH gave me an average of about 4 ppm. The biggest problem is the difficulty of determining an accurate PH for each sample. Try it for some amusement! Mathematically, going down 1.0 in PH has to mean the CO2 increases by a factor of ten, so it all depends on the degassed ppm of CO2 for determining how much that is in ppm in the tank.

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post #8 of 10 (permalink) Old 04-16-2006, 07:13 PM
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Quote:
Originally Posted by Hoppy
The problem is, at 7.8 PH, the KH will not be 3.5, because the amount of CO2 in the water will be about 3 to 4 ppm, not 1.7 ppm. The method of using 1.0 PH drop to get 30 to 40 ppm of CO2 is based on the deaerated water having about 3 to 4 ppm of CO2 in it, which my testing showed it to have (but, that might not be true for someone at way above sea level).

FYI. I am also a mile high here in Albuquerque. I am not sure if that effects the amount of co2 in deaerated water or not. I do know in my case the 1 ph drop rule does not work but the chart does seem to work if I input the actual bi carb component for my KH.

I did look at the chart some more. it certainly does appear if you go down the ph column to find a co2 = about 3 for any ph reading you can use a 1 drop to get near 30ppm CO2. Might work for most but not for everyone.

Rick
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post #9 of 10 (permalink) Old 04-17-2006, 11:08 PM Thread Starter
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Thank you for all the help.

Dan

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post #10 of 10 (permalink) Old 04-18-2006, 12:24 AM
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if you're using a test kit that tells you to count drops and then multiply by ten, the result is in mg/L (a.k.a. ppm). You need to convert the result in mg/L to °KH (which is what the charts use). Here's the formula:

mg/L * 0.056 = °KH

53.7 * 0.056 = 3.0072
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