How much does CO2 drop your pH? - The Planted Tank Forum
 
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post #1 of 13 (permalink) Old 03-27-2015, 02:17 AM Thread Starter
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How much does CO2 drop your pH?

So while at the fish store, they quickly tested my pH-water sample out of my FW tank for diagnosis of snail issues, and I was honestly in awe when he said wow is your water soft... a pH of 6.8?!? I don't usually test my pH and I know in my low tech tank its sitting at 7.4-7.6 (API color card is hard for me to read on the pH test), and out of my tap its at the same level. And here I was thinking my CO2 wasn't being diffused efficiently enough.

So out of curiosity, what is your pH with pressurized CO2 vs. your pH without?
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post #2 of 13 (permalink) Old 03-27-2015, 02:23 AM
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1 degree drop. example 7.0 without, 6.0 with. Approximately 30ppm.

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post #3 of 13 (permalink) Old 03-27-2015, 03:21 AM
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7.7 in the morning, pH controller set to maintain 6.48-6.53 - drop of 1.2
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post #4 of 13 (permalink) Old 03-27-2015, 03:55 AM
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Lee , out of curiousity why dont u leave the co2 running 24/7 and have a constant level of PH.
My controller is set t 6.7 and co2 runs 24/7 with on/off periods . I degased a sample or 24h and had 8.0 PH , a 1.3 drop which gives me about 40ppm .
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post #5 of 13 (permalink) Old 03-27-2015, 05:06 AM
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Quote:
Originally Posted by cbucner View Post
Lee , out of curiousity why dont u leave the co2 running 24/7 and have a constant level of PH.
My controller is set t 6.7 and co2 runs 24/7 with on/off periods . I degased a sample or 24h and had 8.0 PH , a 1.3 drop which gives me about 40ppm .
I used to, for that reason. I had a string of deaths in fish that like lots of O2 (Hillstream Loaches, Otos), so I set CO2 on a timer, and a night-time airstone. All great since.
I also like seeing where the pH starts from, at least a couple of times a week - gives a reasonable confirmation that CO2 should be steady from one day/week to the next.
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post #6 of 13 (permalink) Old 03-27-2015, 01:48 PM
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Plus co2 outside of photoperiod is just a waste of gas.
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post #7 of 13 (permalink) Old 03-27-2015, 01:55 PM
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1 degree drop give or take. One thing to keep in mind is that the pH reading may look different at your home and at the store. When using the color comparison charts like we usually use the type of light we view it through can make a difference. Different lights have different colors in them. Stores typically use fluorescent while most homes still have incandescent or CFLs of various color temps.
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post #8 of 13 (permalink) Old 03-27-2015, 02:01 PM
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ph

I inject co2 from 8 in the morning until 7 in the evening. I have a ph controller. It reads 7.4 in the morning when I turn the co2 on. Then I drop it to 6.5 during the lighted period. So I guess I have a normal ph of 7.4.

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Originally Posted by cbucner View Post
Lee , out of curiousity why dont u leave the co2 running 24/7 and have a constant level of PH.
My controller is set t 6.7 and co2 runs 24/7 with on/off periods . I degased a sample or 24h and had 8.0 PH , a 1.3 drop which gives me about 40ppm .
I like to turn mine off at night to get a degassed reading the next morning, so I can keep the correct drop i need.


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Last edited by rick dale; 03-27-2015 at 02:01 PM. Reason: edit
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post #9 of 13 (permalink) Old 03-27-2015, 02:11 PM
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Co2 turns on at 4, lights come on at 5, co2 shuts off at 10, lights shut off at 12.
I'm still increasing co2 as we speak to get right amount, but right now, 6.96 > 5.77
I'm guessing this is due to offgas and sump and overflow losing co2.
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post #10 of 13 (permalink) Old 03-27-2015, 02:33 PM
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roughly a 1.2 drop


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post #11 of 13 (permalink) Old 03-28-2015, 06:31 PM
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I got the wasteof gass thing but it doesnt run all the time at night time , the controller turns it off when it gets to 6.7 in my case. The way I look at it is I want to have the optimal amount of co2 when the lights turn on.
Question, how long it takes for u guys that turn it off at night to reach your desired ph once it turns on.
I check my ph once a week with the degas method and always had a 1.3 drop , no variance on the initial ph.
I guess it all depends on how fast u can achive your ph goal.
On a 125 gal tank to reach a 1.3 drop if I will turn it off at night will take a really long time so by the time I reach the optimal ph I will only have a few hours of photoperiod at optimal co2 if I will reach it at all during my 8h photoperiod.
When having water surface agitation like mine it is really hard to get that drop but again I dont dump a whole 20lbs tank in one month.
I curious to see what others think about this.
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post #12 of 13 (permalink) Old 03-28-2015, 09:20 PM
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To confirm the 1.0 pH drop will reach CO2=30ppm, some quick math:

CO2 (ppm) = 3 x kH x 10 ^ (7-ph)

Assume a starting CO2 of 3ppm, and target CO2 of 30ppm, sets up two equations with two unknowns, and simplifies to:
10 * 10 ^ (7 - ph1) = 10 ^ (7 - ph2); where ph1 is starting ph, ph2 is target.
Take log of each side:
log10 + (7-ph1) = 7-ph2
and finally ph1 - ph2 = log10 = 1.0

If your starting CO2 level is 3.0ppm, then lowering the pH by 1.0 will give you CO2 of 30ppm. Notice that this is independent of kH. If you are dropping more than 1.0, then either you started less than CO2=3ppm, and/or you are achieving higher than 30ppm.
The kH does matter in terms of where your starting ph is (high kh = high starting pH, low kh = lower starting ph).

Evaluating the CO2 savings from turning off CO2 in the night;
The apex graph from andy gives a great illustration of the gas-off rate vs. the gas-in rate. I'd be curious how much surface agitation his system has. The amount of time the CO2 will stay on depends upon the gas-off rate. In his case, the gas-off rate is quite slow compared to the gas-in rate, so the controller would only kick in for a few hours/night (not bad). But if the gas-off rate was steeper, the CO2 would be on much longer in the night (not so good if you want to conserve gas).
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post #13 of 13 (permalink) Old 03-28-2015, 11:09 PM
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Quote:
Originally Posted by cbucner View Post
Question, how long it takes for u guys that turn it off at night to reach your desired ph once it turns on.
I check my ph once a week with the degas method and always had a 1.3 drop , no variance on the initial ph.
I guess it all depends on how fast u can achive your ph goal.
On a 125 gal tank to reach a 1.3 drop if I will turn it off at night will take a really long time so by the time I reach the optimal ph I will only have a few hours of photoperiod at optimal co2 if I will reach it at all during my 8h photoperiod.
.....
I have lights on at 1200, well - they reach 10% by 1300 and 60% (or max) by 1400. Lights begin to fade at 1900, off at 2100.

I have CO2 turn on at 0900 - pH will be at 6.5 usually by 1100 or so.

The size of your tank shouldn't really matter, bigger tanks need higher CO2 flow/diffusion to compensate.
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