1. Junior Poster
Join Date
Feb 2012
Posts
2

## Passive CO2 enrichment?

I've always wondered about the lengths that aquarists go to insert CO2 into their systems, and the inherent risks that both Pressurized CO2 tanks and Glutaraldehyde (Excel) carry with them.

What I find interesting is that we add all of this complexity to the system to emulate a natural system which is much more elegant. Aquatic plants in the wild do just fine without the luxury of CO2 enrichment. If I understand correctly it is because an planted tank represents a plant density which rivers and lakes will never approach, so the ratio of surface area to plant biomass is much much higher in nature which keeps sufficient CO2 in the system simply from gas exchange (which is limited by surface area).

With that assumption, has anyone ever tried to artificially increase the surface area of their tank until the passive gas exchange was enough to maintain healthy CO2 levels?

I'm no chemist, so I don't know how much you would have to increase surface area to start seeing a measurable effect. However I would think it would be relatively simple to double or triple the surface area in your system without taking up much more space. If you had a 40 gallon breeder, and you plumbed (carefully) an overflow to take the water from the main tank into a 10 gallon tank with shallow trays spaced every half inch you would increase the surface area of your system by a factor of 7 or so. Seeing as how a 40 gallon breeder is 36x18=648 square inches of surface area, and a 10 gallon has a 20x10=200 square inch surface area with a depth of 12 inches which would accommodate 24 half inch trays for a total of 4800 additional square inches of surface area.

Its really no different from plumbing up a very strange sump, its eminently doable and no stranger than the sort of stuff they dream up on the Saltwater side of the hobby. Heck it may be overkill to even glue acrylic trays into a tank, you might be able to get away with placing a large extremely fine sponge in the supplementary tank and just running water slowly through that.

Does this concept seem like it may be valid? The only thing I can think of is that I might be vastly underestimating the additional surface area required to get this to work, but it really seems doable to VASTLY increase the surface area of the tank with very little additional space devoted to the setup.

2. ## No, The Concept Is Not Valid, The Underlying Assumptions Are Incorrect

Hi,

Welcome.

The easiest and most effective way to increase surface area is to increase surface agitation.

The maximum amount of atmospheric Carbon dioxide that can be dissolved in the water is governed by the partial pressure of CO2(gas) in the atmosphere. Increased agitation of the surface can improve gas exchange put cannot increase the amount of dissolved gases in the solvent (water).

Given that approximately 0.03% of our atmosphere is made up of CO2(gas) it will not surprise you learn that 3-ppm CO2(aq) is the “natural” amount in our aquariums and changing the size or shape of the container does not change that amount.

There are many bodies of water that have elevated levels of CO2(aq) “injected” usually via complex water delivery systems through limestone.

There is nothing fundamentally unhealthy about “natural” CO2(aq) levels, there are plants that with or without regard to plant density that simply grow better at elevated nutrient levels. Since the mass of plants are largely made up of carbon many would reason that providing extra carbon in a plant friendly form would be a good idea.

I have tanks that I elevate the CO2(aq) levels by inoculating the substrate with all kinds of critters and pump a lot of air into the tank to keep the O2(aq) high that in turn elevates the CO2(aq).

I also keep quite a number of densely planted tanks with no added CO2; they do very well, thank-you very much.

I also keep a couple of tanks that I raise the
CO2(aq) by breath.

In the greenhouse I keep tanks at elevted levels by raising the atmospheric
CO2 to between .12-.15%.

Biollante
Last edited by Biollante; 02-17-2012 at 08:10 AM. Reason: Format

3. Hi,

Bio is way smarter than I am, but I will for sure support the notion that your basic assumptions are not valid. Natural systems CANNOT be compared to any type of setup. If it could be done easily, we would not NEED c02 or excel, etc. Plant bio-mass in natural waters can far exceed the mass in our tanks. Even seen a hydrilla filled canal in FL???

Many examples exist....

4. Hi Bio, missed you. You too G.

Increased agitation of the surface can improve gas exchange put cannot increase the amount of dissolved gases in the solvent (water).
Is this true for DO levels?

Like a treadmill, only for water.
The easiest and most effective way to increase surface area is to increase surface agitation.

5. Originally Posted by Gierling
I've always wondered about the lengths that aquarists go to insert CO2 into their systems, and the inherent risks that both Pressurized CO2 tanks and Glutaraldehyde (Excel) carry with them.

What I find interesting is that we add all of this complexity to the system to emulate a natural system which is much more elegant. Aquatic plants in the wild do just fine without the luxury of CO2 enrichment. If I understand correctly it is because an planted tank represents a plant density which rivers and lakes will never approach, so the ratio of surface area to plant biomass is much much higher in nature which keeps sufficient CO2 in the system simply from gas exchange (which is limited by surface area).

With that assumption, has anyone ever tried to artificially increase the surface area of their tank until the passive gas exchange was enough to maintain healthy CO2 levels?

I'm no chemist, so I don't know how much you would have to increase surface area to start seeing a measurable effect. However I would think it would be relatively simple to double or triple the surface area in your system without taking up much more space. If you had a 40 gallon breeder, and you plumbed (carefully) an overflow to take the water from the main tank into a 10 gallon tank with shallow trays spaced every half inch you would increase the surface area of your system by a factor of 7 or so. Seeing as how a 40 gallon breeder is 36x18=648 square inches of surface area, and a 10 gallon has a 20x10=200 square inch surface area with a depth of 12 inches which would accommodate 24 half inch trays for a total of 4800 additional square inches of surface area.

Its really no different from plumbing up a very strange sump, its eminently doable and no stranger than the sort of stuff they dream up on the Saltwater side of the hobby. Heck it may be overkill to even glue acrylic trays into a tank, you might be able to get away with placing a large extremely fine sponge in the supplementary tank and just running water slowly through that.

Does this concept seem like it may be valid? The only thing I can think of is that I might be vastly underestimating the additional surface area required to get this to work, but it really seems doable to VASTLY increase the surface area of the tank with very little additional space devoted to the setup.
Natural systems loaded with aquatic plants are CO2 enriched from cave groundwater.
Whether we add the gas is really a simple issue of practical matters.
A tank will have emergent growth as you increase the Surface to volume ratio.
So simply using emergent growth to start with is likely a better idea.

Surface area and exchange rates can be increased using a wet/dry trickle filter..........but.....the concentration in the water will never get about 2-3ppm.

Which is not much.

6. Is that it? 2-3ppm, wow. Do you get an increase in Oxygen levels in tanks with Plants? We should, right?
Last edited by Tug; 02-17-2012 at 09:30 PM. Reason: because

Join Date
Jun 2011
Posts
156
lol maybe its just me, but I would love for biollante to explain "some I raise the CO2 level by breath"

I have often wondered, what the % of air we breath out is CO2

I have a little CO2 setup in one of my tanks that just hold air underwater and lets the CO2 disolve

if my breath is mostly CO2, why would I not just breath into the thing a coupla times a day, instead of a yeast based, system??

if this works why wouldnt pretty much anyone without a pressuized system do this on all their planted tanks, even if only once a day if it works, I mean we may not all be capable of the same things, but I think anyone who can read this can also breath lol....

8. Originally Posted by Tug
Is that it? 2-3ppm, wow. Do you get an increase in Oxygen levels in tanks with Plants? We should, right?
Well, that's what I got from my CO2 measurements, which is in agreement with many other folks. We all might be wrong though.
If so, we are not wrong by much..........Typically about 1-2 ppm higher if you use a Wet/dry consistently over the 24 day cycle.

This was true for a non CO2 and CO2 enriched tank regarding O2.

9. Originally Posted by Green Thumb Aquatics
lol maybe its just me, but I would love for biollante to explain "some I raise the CO2 level by breath"

I have often wondered, what the % of air we breath out is CO2

I have a little CO2 setup in one of my tanks that just hold air underwater and lets the CO2 disolve

if my breath is mostly CO2, why would I not just breath into the thing a coupla times a day, instead of a yeast based, system??

if this works why wouldnt pretty much anyone without a pressuized system do this on all their planted tanks, even if only once a day if it works, I mean we may not all be capable of the same things, but I think anyone who can read this can also breath lol....
Paul K had suggested using your exhalations into a trash bag, then add an air pump to slowly add the CO2 to a tank. But that's not the most practical method unless you breath into a bag often.

Our lungs and heart are actually fair poor regarding efficacy of O2/CO2 transfer.
You realize that all the O2/CO2 we breath..........is converted into dissolved liquid form into the blood via the lungs?
Yep, and we have CO2/HCO3 pH controlled that we often use for aquariums to measure O2/CO2 levels for human health Sciences.

Our lungs can only pull and exhale roughly 50% max of the gas/air we breath. Our hearts are fairly decent(4 chambers, better than 3, 2, or single chambers), but the lungs are not the best.
Birds have a really cool heart lung system though, they exhale 90% of the air! Have you ever seen an "out breath" bird? Suckers fly and go forever.

Natural CO2 rich springs:

Bonita springs in Brasil:

Florida:

NZ:

There are many many examples of CO2 rich natural systems and guess what? They are loaded with aquatic plants.

Not just 1-2 species, but many species..........

10. Junior Poster
Join Date
Feb 2012
Posts
2
Ahh, well I see now that my assumptions were wrong, and I thank you for the enlightenment.

I was operating on the assumption that Aquarium plants used co2 to the limits of their environment, which were dictated by the capacity of a given body of water to absorb it from the atmosphere. I didn't know that it was largely a product of the partial pressure of co2 in the atmosphere and that the surface area/bioload ratio was unimportant in dictating the total dissolved co2.. Hence why I thought that if you could increase the surface area then you could increase the available co2 for the plants.

Although it is very interesting to see that there are co2 enriched environs in nature, although I don't think its very applicable to the aquarium. I guess tossing a big chunk of limestone into ones tank is a less then ideal way of increasing co2.

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