how stable can CO2 be without a pH controller? - The Planted Tank Forum

 
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post #1 of 8 (permalink) Old 07-30-2015, 05:11 PM Thread Starter
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how stable can CO2 be without a pH controller?

The more I mess around with CO2, the more variables I discover that influence dissolved ppm in the water. Most of these variables would seem to change over time on their own.

-current effectiveness of the device responsible for dissolving co2 into the tank
-flow throughout tank
-leaks in the system
-co2 bubbles escaping to the surface
-amount of plant mass, and how fast plants are growing
-water temperature
-off gassing
-fish respiration?
-photoperiod


Given all of these variables, is it even possible to keep CO2 stable without an electronic controller? Has anyone measured ppm (via pH/KH) over time to test this?
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post #2 of 8 (permalink) Old 07-30-2015, 05:36 PM
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If by stable you mean the same reading 24/7, I think it would be better to shoot for a high during the photoperiod and a low at night, to take respiration and O2 production into account. Otherwise, yes if that makes sense!
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post #3 of 8 (permalink) Old 07-30-2015, 06:20 PM
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You also left off one that may effect the CO2 count a bunch?
If you were able to theoretically get a totally stable CO2 PPM in the tank, what happens to that PPM when you do a 25 or 50% water change?
Co2 levels drift and change all the time. Can we measure it closely enough to know it with the hobby level equipment we use?
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post #4 of 8 (permalink) Old 07-30-2015, 07:23 PM
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I've measured actual CO2 levels in a low tech dirt tank. The kH/pH relationship can be misleading outside a controlled condition.

Here's the data
Quote:
Serial Up!

9:24:59 -
Co2 ppm = 18.70

10:0:0 - lights ON!
Co2 ppm = 18.64

11:0:0 -
Co2 ppm = 18.03

12:0:0 -
Co2 ppm = 17.48

13:0:1 -
Co2 ppm = 16.76

14:0:0 -
Co2 ppm = 16.22

15:0:1 -
Co2 ppm = 15.81

16:0:1 -
Co2 ppm = 15.61

17:0:0 -
Co2 ppm = 15.43

18:0:0 - lights OFF!
Co2 ppm = 15.33

19:0:1 -
Co2 ppm = 15.61

20:0:0 -
Co2 ppm = 16.01

Notice how the lights affect the CO2. Plants are actively absorbing the CO2 and CO2 is replenished by the bacteria in the substrate, maybe a little by the fish respiration.

You'd want to look at one variable at a time. It's like herding cats if you take them all at once.
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post #5 of 8 (permalink) Old 07-30-2015, 08:14 PM
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Your over thinking the whole co2 thing. The ideal is to supply the plants with enough co2 to be non limiting. The magic number seems to be 30ish ppm. If you supplying a bit more or a bit less throughout the day I highly doubt the plants will suffer because of it.

To constantly maintain perfect conditions for perfect plant growth is not something we should aim for as things fluctuate too much as you mention in your post.
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post #6 of 8 (permalink) Old 07-31-2015, 04:00 PM Thread Starter
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Quote:
If by stable you mean the same reading 24/7
I guess I don't mean stable within a day/night cycle, but stable over weeks or months. But you bring up a good point, maybe CO2 should actually be "allowed" to fluctuate during the day/night, maybe that's "better" than holding a constant ppm.

Where I am going is that it's tricky to dial in the right amount of CO2 (bpm), and once that's done, how often do we need to re-check the system? Maybe you have things configured such that the drop checker is green (30ppm), but then you do some trimming (removed plant mass) and the next day, the drop checker is yellow and you've gassed fish. Or the opposite, the water level evaporates to below the filter outlet and suddenly more CO2 is being off-gassed...leading to algae!


Quote:
what happens to that PPM when you do a 25 or 50% water change?
I imagine that within a few hours it's back to where it was, but really I don't know! I haven't heard of this being an issue.

Quote:
I've measured actual CO2 levels in a low tech dirt tank.
Thanks! Interesting to see the drop during the day and immediate rise after lights-off. Any chance you've taken measurements spread out over several days or weeks?
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post #7 of 8 (permalink) Old 07-31-2015, 10:12 PM
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A CO2 controller just maintains a constant pH of the water, not necessarily a constant ppm of CO2. KH can slowly decrease or increase, changing the ppm of CO2 when you keep the pH constant. The pH probe can drift out of calibration, changing the actual pH of the water. If you want to maintain the same ppm of CO2 in the water it should be most accurate to just maintain the exact same CO2 bubble rate, and use a CO2 diffusing method that doesn't change in efficiency over time.

Hoppy
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post #8 of 8 (permalink) Old 08-03-2015, 03:20 PM Thread Starter
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Quote:
Originally Posted by Hoppy View Post
If you want to maintain the same ppm of CO2 in the water it should be most accurate to just maintain the exact same CO2 bubble rate, and use a CO2 diffusing method that doesn't change in efficiency over time.
Thanks Hoppy. I will take your word for it!
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