Optimal CO2 levels... a perspecive? - The Planted Tank Forum
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post #1 of 3 (permalink) Old 12-14-2008, 07:05 PM Thread Starter
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Optimal CO2 levels... a perspecive?

On a prior thread, the discussion came up about CO2 levels, and although I couldn't find anything relevant on the web regarding aquatic plants. I did however find some interesting articles regarding terrestrial plants and wanted to share them here...

"Levels of 800 - 1800 ppm have proven to be optimal for the majority of crops grown under protected cultivation and having CO2 monitoring equipment then becomes important to make sure this level is reached and maintained. CO2 enrichment"


By adding CO2 to the atmosphere around the plant, a 40% crop increase was achieved. Whereas previous crops averaged 22 heads per basket, lettuce grown in the increased CO2 atmosphere (550 ppm) averaged 16 heads of better quality per basket.

CO2 levels to 550 ppm produced an obvious increase in yield (over 30%), but the greatest benefits were earlier flowering (up to 2 weeks) with an increased percentage of dry matter.

The addition of controlled carbon dioxide provided a remarkable improvement in blossom quality, number and yield. Plants consistently produced many more flowers with 24 to 30 inch stems. Average yield was increased by 39.7%.

Work in experimental stations has shown that crop increases of as much as 29% have been obtained by increasing the CO2 concentration. "

There is also some more scientific work to support this here.

What is of further interest, is that you can over do it... "Elevation of [CO2] from 1000 to 10000 micromoles mol-1 decreased seed yield (by 37%), harvest index (by 14%)"

OK, now all that said for TPs, what does this mean for APs?

Many of us search for that "sweet" spot CO2 levels. The above states that for Terrestrial plants, that sweet-spot is about 500-800ppm with an average increase in growth in the 30-40% range, and diminishing returns on those increases start happening from about 1000ppm (1 ppt) onwards. This could also explain why many green-house AP growers grow emmersed (so they can benefit from CO2 - an unvalidated assumption).

However, these terrestrial numbers seem to be "killers" of fish in any planted tank - by about a factor of 5 (my estimate, based on several fish-killing experiences).

CO2 seems to be the "magic" we all stand-by to control algae and increase growth in our new-age, thermo-nuclear lighting set-ups. CO2 first... ferts a honorable second.

Whenever we get algae outbreaks, we can generally point to something that's happened, but to control it, we always answer: "Increase the CO2" - either directly (more bps from our CO2 tanks) or indirectly (via increased flow of the CO2 already in the PT). We always seem to be balancing on that thin razor edge of not enough, and too much. I'm coming to the conclusion that that "edge" is much wider than we really perceive it is.

Let me talk about my own very recent observations:
I recently changed out my 10K HQIs for some 8K HQIs... and noticed a significant increase in light output (I'll say visible, as I had them side/side, but no real scientific way of measuring PAR/Lumens, etc...).

Almost immediately, BOOM. Green thread algae started growing at a visible pace. My initial reaction was to cut-back on the photoperiod and increase the CO2. I was also working with a CO2 degassing issue with my sump that wasn't very helpful in getting the levels up, and working through reactor issues as well. It became more frustrating than fun there for a while as no matter what I did, I could not keep the levels down.

As I worked through them, I was eventually able to get my CO2 levels to stabilize at around (what I thought was) 6.4 @4dHk (~50ppm) ... then I looked into the tank and noticed several GBRs on their sides... wth? Turns out, my probe hadn't been calibrated in a (long) while, and the levels were more like [email protected] (~300 ppm). As I was "backing off" and the fish came around (I lost only two), I was somewhere on that "razor's edge" of seeing how much CO2 I could put in the tank without killing the fish and expecting a significant increase in plant growth at those levels (I'm estimating the backing off took it to ~150 PPM). Surprisingly, It didn't (unscientifically) give me massive daily growth and that champagne pearling effect...

In recent days, I've now backed off even further (and recognizing there are many other variables to take into consideration I'm not measuring) and I seemed to have hit that (my?) "sweet-spot" as my sword just put out a leaf in the last three days about 3-3 1/2" wide (double in size of already existing leaves). IIRC correctly (why my laziness of not keeping a written journal would be helpful) I saw this too on my 55G a few years back when I was dinking around with an algae outbreak...

My current levels now are 6.3-6.4 (calibrated ) @ 4dKh (somewhere between 50 and 60ppm).

Now a couple (more) disclaimers - I realize I'm using the pH table/Calculator to arrive at these values, but short of $2500 worth of equipment a new CO2 meter just isn't on my xmas list... (Tom, can I borrow yours ).

So, just some commentary/opinions and questions wrapping up:
1) I seem to get best plant growth in the 50-60 ppm range, and things slow down after that in either direction.
2) Fish start dying in the 150 and up range - and it really doesn't help the plants (from what I can tell).
3) There is a lot of margin we have to go through between getting really good plant growth, and killing fish - we don't need to have the fish anywhere near gasping for air or on their sides on the bottom before we start backing them off... I do have a DC, and it shows me three colors: Green, Blue and Yellow. And once it's yellow, it doesn't get much (perceptibly) yellower as the CO2 increases. I'm seriously thinking of putting a much-higher dKh solution in the DC so that change from green-to-yellow happens at around 70ppm. For that also, I think there is value in actually measuring and knowing your actual pH and dKh values... and the DC isn't a cure-all-end-all. Experience is, but those with less should also KNOW about where they are on the CO2 curve.
4) Lastly, Can anyone find/point-to any scientific work for APs - I understand that we're a small market, so documented work in this area may be hard to find. One benefit of the Internet, is that we can communicate with each other and "unscientifically" share what we see as "optimum" - as well as communicate where we are keeping our tanks pH/dKh wise... (the Calculator, for all of our questions about it's accuracy is pretty-much all we've got).

- Jeff
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post #2 of 3 (permalink) Old 12-14-2008, 09:40 PM
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Let's straighten you out here.
First, units.

For CO2:
Mol/L=> ppms require a conversion of 4.4 X 10^4
ppms => Mol/L requires a conversion factor of 2.273X10-^3

Stay away from air concentrations.
I think if you had about 1800ppm in the air and bubbled that directly into water, you'd get about 25 ppm of CO2 in solution.

Something like that or somewhat close anyway. I did that calcuation many years ago.

More to the point, look at Van et al (Haller, Bowes):


See figures 4. Each is about peaked out around .45 to .5 of so millimol, converting to ppm's, this is about 22 to 30ppm.

This shows for 3 species of aquatic weeds what the saturation CO2 is for non limiting light/Nutrients etc.

Now you have a baseline, it's about 25-30ppm for any light.
Note, not all 300-400 species of plants will be the same, these are fast growing highly adapt weeds, not all plants are like this nor only have 2-3 cell layers of tissue to diffuse through.

So they will max out at a lower ppm value than many species.
Also, today we have much better O2 and CO2 in situ measurement devices, such data was a lot of labor back then.

A wider range of plant light, CO2 and O2 evolving characters would be very interesting for a wide range of species. But that likely answers more than all the stuff you suffered through looking at

Been there.

It is also imprtant to note this paper for another reason, it shows that the light is about 600 micromol or less for maxing out the growth rates. So now you have an upper bound for light under non limiting CO2/nutrient levels.

But again, this nis for these 3 nasty weeds which are really well adapted to higher light and really low light. We might not want to assume much there, but overall, aquatic plants are low light weeds.

Sun light at noon in the summer is about 2000 micromol.

So you have 2 very important parameters in that one paper using 3 different species. Excellent paper for the hobbyist.

Tom Barr

Tom Barr
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post #3 of 3 (permalink) Old 12-15-2008, 03:02 PM Thread Starter
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I think you (mostly) overlooked the points being made which are,

1) CO2 has an optimum value in Air for plant growth - the article you found highlights that for three species is 20<30ppm.

2) There is an upper bound, at least for terrestrial plants (the article I was pointing at that used uMol - I was not using the values, as much as I was pointing out that there is a point of diminishing returns - so much so that growth begins reducing). The question is, where is this value for aquatic plants?

3) That the on-going guidance of "More CO2!" is really only valid up-to the "maximizing value" (Say, 30ppm). We don't need 10x that - or even close to that - to see that our fish are gasping for air. If your fish are gasping for air (or rolling on their sides), you've got WAY too much...

CO2 tends to be the cure-all recommendation for algae, but I think way too many people put way too much CO2 into their tanks - much more than they need to control algae in our thermonuclear lit set-ups. And that I've seen better results when I've backed off significantly from "the edge".
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