How To Test CO2 - The Planted Tank Forum
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post #1 of 22 (permalink) Old 09-26-2015, 11:54 AM Thread Starter
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How To Test CO2

Would you please share with me how you test the CO2 levels in your freshwater tanks? Thanks.
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post #2 of 22 (permalink) Old 09-26-2015, 12:35 PM
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IMO de-gassing is better.
Sample water in tank with pH probe.
Remove a sample and de-gas with an airstone 30-60 minutes.
Read sample with pH probe.
Every 1.0 change in pH is about 30ppm.


another method with calculation would be,
where KH is # of drops and pH reading.
Use scientific calculator on phone.
This method is the solution for all the charts you will find.
It can be very inaccurate if you are mixing your own water.
As in using R/O with lots of additives.
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post #3 of 22 (permalink) Old 09-26-2015, 01:29 PM
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You could also use a CO2 drop checker. They are cheap, and they look pretty cool in the tank.
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post #4 of 22 (permalink) Old 09-26-2015, 02:00 PM
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Drop checker and bubble counter are kinda like dashboard lights.
Just a visual that it is working, unknown is how well.


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post #5 of 22 (permalink) Old 09-26-2015, 02:44 PM
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The CO2 calculator is misleading. There are lots of things that influence the kH & pH besides CO2 in an aquarium. It works great in an empty tank of water.
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post #6 of 22 (permalink) Old 09-26-2015, 03:42 PM Thread Starter
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Quote:
Originally Posted by mistergreen View Post
The CO2 calculator is misleading. There are lots of things that influence the kH & pH besides CO2 in an aquarium. It works great in an empty tank of water.
Say I only have plants, fish and tap water in my tanks, nothing else. Do you think the CO2 calculator would be accurate? If not, how is it measured accurately?
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post #7 of 22 (permalink) Old 09-26-2015, 03:58 PM
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The PH drop is the most accurate that I know of. Too many variables with the KH/PH charts and drop checkers are hard to judge color very accurately. If you use a PH pen it is very accurate, test tube kit not very.
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post #8 of 22 (permalink) Old 09-26-2015, 04:46 PM Thread Starter
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Originally Posted by keymastr View Post
The PH drop is the most accurate that I know of. Too many variables with the KH/PH charts and drop checkers are hard to judge color very accurately. If you use a PH pen it is very accurate, test tube kit not very.
Thanks. That is really helpful. I'll go buy one right away. I hate wasting money on drop tests if they aren't always accurate. I want my results to be credible. dKH testing keeps me wondering about the reality of those tanks. So your method is pH pen and de-gassing in a separate aerated tank?
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post #9 of 22 (permalink) Old 09-26-2015, 08:16 PM
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Yeah, I just put a cup of tank water in a jar and use an airstone for 30 minutes or so. That drives off all the co2 except what is naturally in the atmosphere, roughly 3 ppm. That will be the real unaltered PH of the tank. Then just check the PH of the tank after your co2 has been running for awhile. There should be at least a 1.0 drop in PH, 7.5 to 6.5 for example. Mine goes from 8.2 to 7.0
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post #10 of 22 (permalink) Old 09-26-2015, 08:58 PM
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Fluval drop checker is easy, If the liquid in the thing(background is white anyways) is

dark blue - low CO2
dark green - some CO2
light green towards yellow - lot of CO2
yellow -Stop, Too much CO2

But again, it takes time so first few days, do not adjust anything. try and learn the behaviour of the liquid. Soon you will be able to predict the color according to what you do to CO2 injection. Hope this helps.


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post #11 of 22 (permalink) Old 09-27-2015, 04:52 AM
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Knowing that the equilibrium concentration of CO2 in water exposed to the atmosphere is "about 3 ppm" doesn't really let us accurately determine how much CO2 we have in the water when we add CO2 to our tank. "About 3 ppm" might be 2.2 to 3.8, for example. A pH drop of 1.0, means we increased the amount of CO2 in the water by a factor of 10 (10 to the change in pH power) But that would mean our water has 22 to 38 ppm of CO2 - not very accurate. But, we can't measure pH much more accurately than +/- 0.2, so our 1 pH drop might be as little at 0.8 or as much as 1.2, and 10 to those powers is 6.3 and 15.8, so the real range of CO2 concentrations is 6.3 x 2.2 = 14ppm to 15.8 x 3.8 =60ppm which is now a useless measurement. Granted, you can possibly do a good enough job calibrating your pH probe so the accuracy is better than +/-0.2, but you will always have an error range greater than zero.

I think it is best to just assume we can't measure the ppm of CO2 in the water. What we can do is use a drop checker to tell us that we do have a significant amount of added CO2 in the water (we aren't just leaking the CO2 or degassing it rapidly). Then we can slightly increase the bubble rate of the CO2 and watch the fish and plants. We can repeat that until either the plants don't grow any better with the last change, or the fish are showing too much misery from the CO2. Back off the bubble rate to the previous rate and that is the optimum amount of CO2 for your tank, with your plant load, your lights, your oxygenation of the tank water, your fish, etc.

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post #12 of 22 (permalink) Old 09-27-2015, 05:22 AM
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An excellent summary Hoppy.

Adding to the top paragraph, there is also some concern regarding 1.0 pH change represents a 10x change in CO2 concentration.

https://en.wikipedia.org/wiki/Carbon...ueous_solution

Quote:
The hydration equilibrium constant of carbonic acid is (at 25 C). Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains as CO2 molecules, not affecting the pH.
I'm not entirely sure yet how this affects the maths, but it is cause for concern.

So your last paragraph is (IMHO) the interesting part in terms of how people should judge CO2 levels in their aquariums, until such time as clarification regarding actual CO2 concentrations has been delivered.

Feel free to edit.

Last edited by Audionut; 09-27-2015 at 05:38 AM. Reason: Remove my stupid math mistakes.
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post #13 of 22 (permalink) Old 09-27-2015, 01:59 PM
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Quote:
Originally Posted by Hoppy View Post
I think it is best to just assume we can't measure the ppm of CO2 in the water.


What we can do is use a drop checker to tell us that we do have a significant amount of added CO2 in the water (we aren't just leaking the CO2 or degassing it rapidly).


Then we can slightly increase the bubble rate of the CO2 and watch the fish and plants. We can repeat that until either the plants don't grow any better with the last change, or the fish are showing too much misery from the CO2. Back off the bubble rate to the previous rate and that is the optimum amount of CO2 for your tank, with your plant load, your lights, your oxygenation of the tank water, your fish, etc.


We are guessing, but at least it is some form of measurement.
Having some way to judge CO2 content is better than none.


We all seem to agree drop checker is a dashboard light.


I am one to judge by fish behavior for optimum CO2 settings.
Keeping simple livebearers I would not feel great loss to lose an occasional fish.
Others in the forum have raised concerns about hurting their fish.
Understanding that some have forked out a lot of $ in fish.
I'm in it for the chaotic jungle effect, a plant collector per say.
If raising expensive fish a CO2 enriched environment would not be my choice.
Proper habitat for the breeding fish would be prime concern.


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post #14 of 22 (permalink) Old 09-27-2015, 02:31 PM
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Ph vs. known water hardness works. You could buy one of these.
Carbon Dioxide Test Kit - 7297-DR-01 - Aquarium and Fish Farming - LaMotte
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post #15 of 22 (permalink) Old 09-27-2015, 03:57 PM
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Quote:
Originally Posted by Steve001 View Post
Ph vs. known water hardness works.
By that reasoning, 60ppm of CO2 is not lethal to fish.

Consider my water. The source water has a KH reading of 1.25 dKH, this is rain water into a tank with a concrete inner lining. So it's likely from limestone, which is CaCO3. But for [censored][censored][censored][censored]s and giggles, lets right that off as some other form of total alkalinity. I raise my water to 3.25 dKH with CaCO3 (shell grit), so we can safely assume that my water contains (atleast) 2 dKH of carbonate hardness.

The tank sits at a pH of 5.8, but for more [censored][censored][censored][censored]s and giggles, lets assume that the dual point calibrated pH meter is reading 0.2pH to low.

3 x 2 x 10 to the power of (7 - 6) = 60ppm.

Feel free to edit.
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