pop's opinion: CARBON DIOXIDE a story - Tropical Fish Keeping - Aquarium fish care and resources
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post #1 of 13 Old 06-07-2013, 08:51 AM Thread Starter
pop's opinion: CARBON DIOXIDE a story

I have plastic plants in the aquarium so I thought there are no concerns about carbon dioxide and carbonate hardness. Issues about CO2 and carbonate hardness only apply to the planted tank and not my situation. I was wrong thinking this, no matter how much I try to ignore notions about CO2 it keeps showing up when considering the natural process that take place in my aquarium.

I have come to view carbon dioxide as the provider of balance and harmony to all process that are driven by oxygen and dissolved oxygen that occur in aquaria. This symbiotic relationship is the basis of life-promoting processes in our tanks. We take the breath of life by breathing air and exhale carbon dioxide as do our tanks inhabitants. Yet this relationship extends far beyond just respiration and is necessary component for a healthy and thriving aquaria community.

One of my ill conceived notions about carbon dioxide as being the limiting factor in the diffusion of dissolved oxygen in the water column led me astray. Once I realized that CO2 had to follow the same process to enter water as does oxygen I began to see balance and the symmetry of the combined process. I thought it’s like a dance of nature where each separate process advances to a state of harmonic equality that provides the underlying structure for the next level of harmony creating equilibrium. So let’s dance with nature to the harmonic music of equilibrium ……….

When oxygen molecule enters the water column it remains an oxygen molecule it doesn’t matter if the molecule is in a state of solute (dissolved oxygen) or a gas state. There is no change in water chemistry, but when carbon dioxide enters water in a dissolved state (solute) or gas state it actively interacts with water mitigating the lack of activity of oxygen molecule bringing equilibrium and change to the water chemistry. What happens is a small portion of CO2 combines with water and forms a temporary solution of carbonic acid that falls apart leaving a (H+) ion this disassociated free hydrogen ion is positive charge (H+) and constantly forms an unstable hydronium ion (H30) positive charged. These charged hydronium ions plays necessary functions in vital acid-based chemistry as moving ions across biological membranes (osmoregulation), regulating cellular acidity and providing energy for photosynthesis again bringing harmonic equilibrium to the aquaria ecology.

In our aquariums these bicarbonate ions (H+) is the carbon dioxide and it has direct affect on carbonate equilibrium effecting pH values. One way pH can be thought of as the ratio between hydrogen ions (H+) and hydroxyl ions (OH-). In water where hydrogen ions are equal to hydroxyl ions the water is neutral, when there are more hydrogen ions the water is acid and when there are more hydroxyl ions the water is basic.
Another way to think of pH is the balance between carbon dioxide (H+ ions) levels and the amount of carbonate hardness (KH), carbon dioxide lowers pH and carbonate hardness raises pH. Calcium and magnesium combine with carbon dioxide to form carbonate hardness (carbonate reserves). Carbonate hardness adds another layer of harmonic equilibrium by stabilizing ever changing pH.

Bicarbonate / carbonate is one side of the benefit carbon dioxide offers. Another side, it is a necessary ingredient for photosynthesis in aquaria plants.

Carbon dioxide has a greater affinity for water than oxygen and easily dissolves in water but very slowly diffuses into the water column when compared to faster diffusion of oxygen balancing to the general process of gaseous exchange. Carbon dioxide’s extremely slow diffusion results in low levels of CO2 in the aquaria.
Plants in the aquarium are faced with additional problem created by the Prandtl boundary a thick unstirred layer of still water that surrounds plant leaves that CO2 and nutrients have to diffuse through in order to reach plant leaves. The thinner this unstirred layer is more easily carbon dioxide is absorbed by the plant leaves increasing the plant ability to complete photosynthesis.

The harmonic equilibrium that carbon dioxide establishes in aquaria is the balancing ecology that we fish keepers seek.

Note: I used the phrase harmonic equilibrium to represent the rhythm of life that you hear when listening to a heart beat or the thunder of swimming fish dancing with nature.


Before the LAW …. pop you are guilty of plagiarizing take him away in chains

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post #2 of 13 Old 06-07-2013, 09:28 AM
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Pop, can you expound a little on the Prandtl boundary ? I'm trying to associate it with water movement in the tank.

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post #3 of 13 Old 06-08-2013, 07:35 AM Thread Starter
hello fishmonger:
Prandtl’s boundray is a theroy that “effect of friction was to cause the fluid immediately adjacent to the surface to stick to the surface—in other words, he assumed the no-slip con-
dition at the surface—and that frictional effects were experienced only in a boundary layer, a thin region near the surface. ……. explanation of the physical
process in the boundary layer between a fluid and a solid body could be obtained by the hypothesis of an adhesion of the
fluid to the walls, that is, by the hypothesis of a zero relative velocity between fluid and wall. If the viscosity was very small and the fluid path along the wall not too long, the fluid velocity ought to resume its normal value at a very short distance from the wall. In the thin transition layer however, the sharp changes of velocity, even with small coefficient of friction, produce marked results. The velocity changes enormously over a very short distance normal to the surface of a body immersed in a flow. In other words, the boundary layer is a region of very large velocity gradients.” (1)

What this means to me is that as water flows around a plants leaves, stems and roots some of the water sticks to the object and ceases to flow creating a boundary that can be thick or thin depending on the current of the moving waters. This boundary effect might be the reason why uneaten food ends up collecting on décor and leaves.

“Prandtl’s principal assumptions are listed below.
1. When a fluid flows past an object at large values of the Reynolds number, the flow region can be divided into two parts.
(i) Away from the surface of the object, viscous effects can be considered negligible, and potential flow can be assumed.
(ii) In a thin region near the surface of the object, called the boundary layer, viscous effects cannot be neglected, and are as important as inertia.
2. The pressure variation can be calculated from the potential flow solution along the surface of the object, neglecting viscous effects altogether, and
assumed to be impressed upon the boundary layer.” (2)

(1)Anderson, John D “Ludwig Prandtl’s Boundary Layer” Physics Today December 2005: Web. 7:22 am;8Jun 2013. <http://www.aps.org/units/dfd/resources/upload/prandtl_vol58no12p42_48.pdf>

(2) Subramanian, R. Shankar “Some Elementary Aspects of Boundary Layer Theory ” Web.7:11 am;8june2013.< http://web2.clarkson.edu/projects/subramanian/ch302/notes/boundaryla.pdf>
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post #4 of 13 Old 06-08-2013, 08:00 AM
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Here's some "out loud" thinking.

The current is typically low enough that unbeaten food would settle on the leaves regardless of any boundary layering, particularly as this layer is thin enough in such a low viscosity fluid as water that it is negligible with regards to food particles.

If you consider the surface of the water and air contact and the gas transfer that occurs there in both directions, depending upon factors such as the current dissolved gas concentration and partial pressures, the transfer is reasonably efficient enough that the water and air reach their equilibriums relatively quickly... say in a 24 hour period.

Now take the boundary layer and surrounding water that is not bound; the transfer between mediums of the same density, same composition but with different CO2 (the typical gas always referred to with plants) concentrations would yield a more efficient transfer. The boundary layer is also not glued in place as it will transfer water molecules, albeit reluctantly.

Also consider that this boundary layer allows a plant leaf to have captured CO2 in place for longer than the moving water may allow which will increase the capability of the plant to be able to absorb the CO2 which increases the disparity between boundary and non-boundary water which serves to increase the transfer between the two. No matter the dynamics involved the transfer will always lean toward equilibrium. It's only a matter of timing.

Putting the settled food particles theory back in for a moment, this would serve to provide captured decaying material which will offgas both CO2 and ammonia right near the leaf. Even though I consider the boundary layer inconsequential in capturing the food I would recognize that it may assist in this manner with smaller organic particles. Unbeaten food will not, in and of itself, block enough light to affect the health of the plant, unless food was really dumped in in which case there will be other more serious imbalances occurring. Other suspended organics would also not impede light on the surface unless some other algae were involved resulting from an imbalance.


Total years fish keeping experience: 7 months, can't start counting in years for a while yet.

The shotgun approach to a planted tank with an LED fixture

Small scale nitrogen cycle with a jar, water and fish food; no substrate, filter etc
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post #5 of 13 Old 06-08-2013, 12:18 PM
While I do agree carbonates and bicarb are very important in buffering an aquarium. The carbon equilibrium is highly pH dependent. Very little CO2 is going to dissociate to carbonic acid in the aquarium. That is why when we planted people inject pressurized CO2 usually up to 30ppm pH drops but KH and GH remain basically the same. And we measure CO2 as CO2 not carbonic acid. Most of the carbonates in a tank are already dissolved in the tap water and come from the bedrock. Due to typical aquarium pH they exist as bicarbonates and carbonates. For carbonic acid to turn to bicarbonates it requires a much lower pH then you will find in a typical aquarium.

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post #6 of 13 Old 06-10-2013, 05:47 AM Thread Starter

Hello JDM:

I don’t understand your point very well but I think I agree some what with your saying. I was taking somewhat different path and was thinking about dissolved and particular organic carbon and bacteria growth that break down organic material releasing carbon to the plant and creating the ecology for carbon and nitrogen fixing bacteria. My thinking is this boundary layer of still water would be the perfect environment for this to happen.

As I understand this layer of still water is thickest in cases where the water movement is the slowest because viscosity impact is greater due to friction. Which I think means this still water is like a magnet attracting all kinds of matter. We are talking about hydro-dynamics which works a little different than reason might indicate.

I must be confused thinking that the gaseous exchange happens instantly and equilibrium is attained for the waters in contact with the atmosphere. The factors limiting this gaseous exchange also include temperature, the actual chemistry of the water, pressure and surface area.

The basic idea of still water boundary is the movement of water molecules is restricted and all elements must diffuse through this layer of water so yes this boundary is glued to the surface of the object. Remember that carbon diffuse very slowly into the water column and has a very, very great affinity for water molecule so much so carbon dioxide has to be stripped from the water molecule.

Thanks for the interesting observation.

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post #7 of 13 Old 06-10-2013, 06:04 AM Thread Starter
Hello Mikaila31:
I have found your post interesting and I agree very much with what you state. But I tend to think that pH is dependent on carbon equilibrium that is the relationship between carbon dioxide and carbonate hardness. I thought that carbon dioxide saturation was the determined by the values of carbon hardness and pH like on a chart.

Help me out here since you know what you are talking about to understand this issue.

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post #8 of 13 Old 06-12-2013, 01:23 PM
Originally Posted by pop View Post
Hello Mikaila31:
I have found your post interesting and I agree very much with what you state. But I tend to think that pH is dependent on carbon equilibrium that is the relationship between carbon dioxide and carbonate hardness. I thought that carbon dioxide saturation was the determined by the values of carbon hardness and pH like on a chart.

Help me out here since you know what you are talking about to understand this issue.

Your correct that CO2 can be calculated on a chart to some degree for the aquarium. This isn't done much anymore since drop checkers came around as charts allowed for too much error. pH is dependent on carbon equilibrium a great deal but not entirely, since more then carbon effects H+ and OH- concentrations. CO2 and carbonic acid are at levels incredibly smaller then bicarb and carbonates. So they don't have as large an effect. CO2 solubility is also effected by temperature and other factors in water. Like everything chemistry related to ecosystems there isn't a clear cut answer because it varies depending on the situation. Rain falling in the amazon is going to increase CO2 levels and its likely a much larger amount of CO2 will be taken up by the water, then say if it fell in the African rift lakes which would likely push it to diffuse back into the atmosphere. Some of the african rift lakes are very unique in relation to CO2 due to where they are located and the chemistry of the water. Some of the more volcanic rift lakes have CO2 artificially pumped out of them by bringing deep water up to the surface.

There simply becomes a point were water becomes CO2 saturated and this almost entirely dependent on pressure. When dealing with surface waters you have to consider the partial pressure of a gas above the water. Partial pressure is the main thing that will determine saturation. If you are dealing with RO water pressure and temperature are really the only factors. CO2 is highly soluble (much more then oxygen) yet it makes up so little of the atmosphere(around 400ppm now) its partial pressure is low so very little of it dissolves into water. A typical aquarium usually holds less then 10ppm of CO2 unless it is injected. Its the same reason so much CO2 is dissolved into a can of soda but soon as you open it and the pressure is removed the CO2 immediately starts gassing out. CO2 also comes from respiration of the the organisms in the tank and breakdown of any organic matter, which is mostly what we care about in a aquarium. Once saturation is reached any additional CO2 dissolved in the water encourages some CO2 to diffuse into the atmosphere. Surface movement encourages CO2 to stay in equilibrium which is why most planted folk shun surface movement and aim to keep it at a minimum.

For CO2 to dissociate into carbonic acid depends on its dissociation constant (pKA) in a solution as well as other properties. It is also dependent on pressure, and at standard pressure little of the CO2 will form carbonic acid(much much less then 1% of the CO2 will dissociate).

There is always some confusion as the way things function in aquariums as the way things function in a complete ecosystem are different. Naturally most lakes, rivers, and oceans are a carbon sink. Meaning they uptake more CO2 then they release. Far as I know our aquariums (planted) often do the opposite, this usually has to do with tanks being highly productive in comparison(more CO2 is generated). They are also incomplete in their nutrient cycles as they simply don't host all the environments needed for them.

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post #9 of 13 Old 06-17-2013, 06:47 AM Thread Starter
Hello Mikaila31:
Your post is very interesting and I want to thank you for taking the time to help me out here. You gave me a lot to think on as the role pressure plays in diffusion of carbon dioxide in the water column. At first I thought that this pressure was equilibrium at surface between water and atmosphere but this is only partially correct the pressure you are focusing on is the results of carbon dioxide acting within the water column. So will this ‘pressure’ act the same with respiration loading up CO2 in water molecules?

One of the places I am going is toward a notion that carbon dioxide becomes stratified into layers determined by diffusion gradient from respiration of water critters and plants. Your assessment of the notion of water being a carbon sink provides a nice stepping stone for this stratification to occur. Since carbon can not be created or destroyed it just moves from one sink to another sink reminds me of photosynthesis (taking up CO2) and respiration (releasing CO2) making plant also a carbon sink. This is a little different than your notion of a sink but means the same thing.

Pressure has been talked about and temperature I guess is next to considered. Why will cold water have a greater potential to hold vast amounts of CO2 than warm water, is it because cold water has a greater surface area for CO2 diffusion. Is this increased surface area determined by temperature one factor in gassing off CO2 as well as the amount of carbon dioxide in the atmosphere?

Because carbon dioxide is a green house gas and in recent times CO2 levels in the atmosphere have been increasing resulting in significant amount of carbon dioxide available for exchange into carbon sinks like our aquarium. This should naturally supplement CO2 values reducing the fear of gassing off this resource because warmer temperatures reduce surface area in contact with the atmosphere and the enhanced levels of CO2 (green house gasses) in the atmosphere limit gaseous exchange.

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post #10 of 13 Old 06-17-2013, 10:56 PM
Respiration acts through diffusion by adding CO2 to the water. The CO2 concentration is higher in the fish then in the water, both are at the same given temperature. The pressure as far as we care is the same in terms of atmosphere or water pressure, the fish or plant will have internal pressure as most all organisms do this will also encourage CO2 to move out of organism and into the water. If the water is already saturated with CO2, respiration will still add CO2 to the water but CO2 will also be encouraged to gas out of the water into the air or to diffuse to areas of lower CO2 levels, as things are always pushed towards equilibrium. Equilibrium is a tricky word, CO2 in surface water and air are at equilibrium at very different levels.

Temperature effects water density. The colder water is the more dense it is to a point(about 40*F). Ice obviously is less dense since it floats. This effect on density it likely why it effects CO2 levels but I am not entirely positive on this.

In an aquarium you typically do not have stratification or much of any diffusion gradient as these are not desirable. A typical aquarium will have too much circulation for these to be significant in such a short depth. The filter turns water over constantly bringing lower water up near the surface. Stratification does happen in large rivers and most ponds and lakes and is an important part of the ecosystem there. However usually one compound, CO2, in this case does not become stratified without the same happening to other compounds and parameters like O2 and temperature. Lakes are typically stratified by temperature and in turn density which impacts the thermodynamics of compounds like CO2. Much of it does not matter in an aquarium as an aquarium is a VERY simple system in comparison. Lakes have depth and once you get away from surface waters then water pressure comes into play not atmosphere(but you have to be deep). The deeper you go the colder the water and the more pressure and the result is higher potential for CO2 saturation at that depth. But just because it can happen does not mean it will. Lower waters do not often get anywhere near saturated with CO2 because the surface is so far away, deep water is typically low light and has low biotic levels so CO2 levels stay low. This however is not what happens with a few infamous rift lakes, but high CO2 levels are due to volcanic activity(large CO2 producers) saturating the lower water layers. The pressure and stratification keeps it there since any upper layers have lower CO2 saturation, until there is a sudden large violent release of the built up gas out of solution. If you are interested in these specific examples you should look up lake Nyos, lake Kivu, and lake Monoun. They are very specific in being the only known lakes where this happens but they are still very interesting in their own right.

Plants are certainly carbon sinks. I'm still hesitant to say aquariums in general are carbon sinks as most are unplanted and certainly not. Most high tech tanks would be since they obviously demand CO2 injection and anything in between would vary. CO2 is really not a concern in the aquarium unless it is a limiting nutrient for aquatic plants. Yes its in the water regardless, but it is little unless you inject it. CO2 rarely becomes limiting in any natural aquatic ecosystem as other nutrients are often limiting before CO2(its often phosphorus). Given the frequent water changes most tanks get the vast majority of carbonates often comes from ground water as it leaches out of bedrock either into a aquifer or into surface water. Also you must be aware increasing CO2 will cause a decrease in O2. Yet obviously some including myself inject it but only to a point. Usually 30ppm is about what you want in a high tech planted aquarium. There is no point in go higher as the benefits are little and there is the risk of killing fish from CO2 injection(often just called gassing them lol). Either way you need the light intensity to demand the extra CO2, if you don't have the demand then it is not limiting anything and there is no point to it at all. That goes for any nutrient.

Aquariums break a lot of rules or situations that happen in a natural ecosystem which is why the notion of a self sustaining tank is always always very far from reach. Aquariums including planted have almost no complete nutrient cycles as any natural lake would have and that includes carbon. Sure plants uptake carbon and many other nutrients in a sink yet once plants grow to a certain point we step in and start removing them and break the cycle at that point.

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