O2/CO2 rant - The Planted Tank Forum
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post #1 of 33 (permalink) Old 01-09-2009, 01:25 PM Thread Starter
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O2/CO2 rant

One meme that keeps popping up is the idea that excess CO2 will "drive off" O2 from the water.

Every chemical that is dissolved in a given volume of water has a maximum concentration. Beyond that concentration it will precipitate out as a solid or a bubble, depending upon the chemical (and in the case of bubbles, on the gas "partial pressure" above the water).

The concentration of any particular chemical does not affect the maximum concentration of any other chemical in that volume of water. In other words, there is nothing you can add to a volume of water that will "drive off" anything else that is in that water.

By the same token, the partial pressure of any gas does not affect the partial pressure of any other gas.

Sometimes when you add two chemicals to the same body of water they recombine to make other chemicals, and these may have different maximum concentrations. However, this is not true for CO2 and O2.

If your fish gasp at the surface when the concentration of CO2 goes too high, it is not because you have "driven off" the oxygen. It is probably because their gills are burning because of carbonic acid. You are exposing them to unnecessary pain and permanently injuring them.

Stop that! (end of rant)

Thanks,
Rod
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post #2 of 33 (permalink) Old 01-09-2009, 02:16 PM
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I sense some baiting going on, but here goes:

Quote:
Originally Posted by houstonhobby View Post
One meme that keeps popping up is the idea that excess CO2 will "drive off" O2 from the water.

Every chemical that is dissolved in a given volume of water has a maximum concentration. Beyond that concentration it will precipitate out as a solid or a bubble, depending upon the chemical (and in the case of bubbles, on the gas "partial pressure" above the water).
Well, that's kind of true. However, temperature and TDS both have a significant effect on solubility.

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Originally Posted by houstonhobby View Post
The concentration of any particular chemical does not affect the maximum concentration of any other chemical in that volume of water. In other words, there is nothing you can add to a volume of water that will "drive off" anything else that is in that water.

By the same token, the partial pressure of any gas does not affect the partial pressure of any other gas.

Sometimes when you add two chemicals to the same body of water they recombine to make other chemicals, and these may have different maximum concentrations. However, this is not true for CO2 and O2.
Well, the first statement above is only true in the most simplistic of terms. For instance, O2 is less soluble in saltwater at the same temp as a sample of DI water - because all the ions in the saltwater DO have an effect. The same is true for volatile organics - the effect is called "salting out".

I'd also discuss the last paragraph more - if we change the pH of a water sample (by adding HCl), we convert the existing bicarbonate to carbonic acid which in turn degasses as CO2.

Quote:
Originally Posted by houstonhobby View Post
If your fish gasp at the surface when the concentration of CO2 goes too high, it is not because you have "driven off" the oxygen. It is probably because their gills are burning because of carbonic acid. You are exposing them to unnecessary pain and permanently injuring them.

Stop that! (end of rant)

Thanks,
Rod

Does carbonic acid in water really burn fish gills??? Since the pH is rarely below 6 in these cases, and some run their tanks with a pH below six and have very healthy fish, I'd argue that is not the case.

Kevin

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2g Mac-quarium. Clown gravel, fluorescent plastic plants, and 2 guppies.
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post #3 of 33 (permalink) Old 01-09-2009, 02:36 PM Thread Starter
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Good points. But none of it refutes the main theme, which is that adding CO2 does not drive off O2. If the fish suffer when you turn up the CO2, it is not because of lack of oxygen. If somebody knows what is causing their gasping I would like to know. Maybe at high concentrations the CO2 interferes with O2 absorption by the gills in the same way that CO would?
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post #4 of 33 (permalink) Old 01-09-2009, 04:52 PM
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If you are injecting co2, then less o2 is in the water because there is a maximum amount of dissolved gas is the water, and there is so much co2 that there is not "room" for much air/o2 to become dissolved.

If you inject o2, then you can inject more co2. I think plantbrain mentioned this, but I could be wrong.

I could be wrong about this whole thing though.

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post #5 of 33 (permalink) Old 01-09-2009, 05:43 PM
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Quote:
Originally Posted by houstonhobby View Post
Good points. But none of it refutes the main theme, which is that adding CO2 does not drive off O2. If the fish suffer when you turn up the CO2, it is not because of lack of oxygen. If somebody knows what is causing their gasping I would like to know. Maybe at high concentrations the CO2 interferes with O2 absorption by the gills in the same way that CO would?
That's correct - I don't see any reason why adding CO2 would significantly affect the O2 concentration.

I suspect the RAPID change is more of the problem - if the fish does not have a chance to adjust physiologically to the new pH then they will have problems with O2 exchange (which is dependent on blood pH). Thus a slow increase in CO2 concentration (over several days) should not be as detrimental to the fish as the same increase over several hours.

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2g Mac-quarium. Clown gravel, fluorescent plastic plants, and 2 guppies.
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post #6 of 33 (permalink) Old 01-09-2009, 07:08 PM
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Quote:
Originally Posted by KevinC View Post
I suspect the RAPID change is more of the problem - if the fish does not have a chance to adjust physiologically to the new pH then they will have problems with O2 exchange (which is dependent on blood pH). Thus a slow increase in CO2 concentration (over several days) should not be as detrimental to the fish as the same increase over several hours.
co2 goes from 4 to 30 in an hour everyday with no harm...

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post #7 of 33 (permalink) Old 01-10-2009, 12:16 AM
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I suggest some reading on the subject of hypercapnia.

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post #8 of 33 (permalink) Old 01-10-2009, 04:12 AM
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Another way to say what houstonhobby's first post said is this (Henry's Law):
http://en.wikipedia.org/wiki/Partial...ility_of_gases

only with lots of scary math symbols and such, things that make some biologists feel like a priest at a heavy metal concert. Note:

"the concentration of a solute gas in a solution is directly proportional to the partial pressure of that gas above the solution." "That gas". Not "all the other gases", or "that gas and oxygen". That gas. Take a sealed container of air (~80% nitrogen, 20% oxygen) and water, replace the nitrogen with CO2 while holding the oxygen at 20%, and it will have diddly squat effect on the amount of O2 dissolved in the water.

I suspect this idea comes from a misunderstanding of why and how we aerate water. Fish are constantly depleting the oxygen from the water surrounding them. Ignoring plants for the moment, this oxygen needs to be replaced by more diffusing into the water from the atmosphere. If the tank is still (no aeration) oxygen must work its way from the surface to the bottom. This takes time, and a heavily stocked tank can go anoxic without further help. To avoid an oxygen deficit we have to bring more of tank water into direct contact with the air during a given period of time. An airstone isn't "injecting oxygen" into a tank; it's sticking little chunks of atmosphere down near the bottom of the tank where there's less dissolved oxygen and a bigger gradient, as well as physically churning bottom water up to the surface where it can grab oxygen on the way by, as it were. Suddenly your tank's surface area has magically tripled or quadrupled.

A fish is constantly producing carbon dioxide as a waste product. In effect, there's a bubble of high CO2/low oxygen water around each fish and that excess CO2 is trying to diffuse through the water column and into the (low CO2) atmosphere in the same way oxygen is travelling in opposite direction as the fish is using it up. A CO2 diffuser is no different than a fish; there will be a bubble of high CO2 water around it. Move that water to the surface and the excess CO2 will diffuse into the atmosphere. The same conditions that encourage oxygen to diffuse into the water also encourages the loss of CO2 out of the water, but the oxygen is not physically displacing the CO2.


So, why are people finding their fish gasping at the surface after a night of CO2 injection? I don't know. One of these days I'll have a chat with a fish physiologist about it. I think there are a couple of possibilities.

Mammals and fish do not use the same physiological mechanisms to control breathing rate. In mammals breathing is stimulated by the level of arterial CO2. If CO2 levels are kept low and oxygen levels are reduced the animal will quietly expire without gasping or distress. Feed that animal a 95% oxygen / 5% CO2 mixture and that same animal will be gasping in distress. This is the classic hyperventilation / hypercapnia response seen in humans. Lots of O2, but too much CO2 at the same time. A mammalian body isn't particularly sensitive to oxygen levels. If it was there would be a lot fewer confined-space deaths as people would have a better idea of when they're in an anoxic environment and need to leave.

Fish on the other hand have oxygen sensors in the gills which respond to levels of environmental O2 and adjust gill motion accordingly. There are also gill receptors which respond to hypercarbia (aquatic equivalent of hypercapnia) but the O2 receptors are supposedly the dominant driving force. As far as I know there are no centralized mammalian-style CO2 receptors in fish. Oxygen levels are far more significant for fish gill movements than mammalian breathing.

It would be interesting to find out how many of those who have had this problem also have their tanks set up to conserve CO2 by using low aeration rates and tight covers. If this is the case then the hypercarbia might well be less of a problem than the possibility the fish run out of oxygen at night while the plants are producing CO2 as well as the fish. Not many of us have a dissolved oxygen meter so we're missing an essential piece of information. CO2 was high, but what was the oxygen level when this happened?

Finally, consider that CO2 is heavier than air. This has led to deaths in industrial situations where workers entered a sump, stairwell, or other low area where CO2 had leaked and pooled. I suspect an aquarium with tight lid and external filter might be the same situation. CO2 bubbled into the tank may have nowhere to disperse to once it reaches the surface and ends up pooling under the cover. Between this and the glass cover the water would effectively be sealed away from the atmosphere, with oxygen unable to diffuse in to where it's needed. Bit like a wax seal on a jar of jam. Only the wax is toxic, as well as anoxic.

I guess what I've persuaded myself of in this wall o' text is that fish gasping at the surface first thing in the morning might still be due to a lack of oxygen, but not through this oft-repeated chestnut of it being driven out of solution by CO2. The fix would still be better aeration, a better ventilated cover, and a timer/solenoid on the CO2. The trick is to keep both the CO2 and O2 levels where they need to be.

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post #9 of 33 (permalink) Old 01-10-2009, 04:55 AM
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The part you seem to be missing is that in the absence of light (ie at night) plants ABSORB O2 and RELEASE CO2 - this is why at night heavily planted tanks can become O2-depleted and why I personally always recommend running an airstone at night to help increase O2 levels and outgas CO2 in CO2-injected tanks.

Back before the days of filters/airstones/water circulation on tanks (if you read some old hobbyist books) people said it was a huge no-no to keep fish and plants together in tanks because of this phenomena at night; the fish suffocated. The technology of putting airstones in tanks to increase circulation/oxygenation was a huge development in the hobby.





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post #10 of 33 (permalink) Old 01-10-2009, 05:51 AM
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I dont know if this helps at all but I thought it would be useful to explain the process by which O2 is exchanged from outside environment to blood to cells and then CO2 in reverse.

As neither is actively transported, that is, no energy is expended in moving it across a membrane such as a capillary in the gills to the outside environment, both CO2 and O2 move down their concentration gradient. This means that O2 and CO2 flow freely from areas of high concentration to areas of lower concentration.

Lets start with O2 from outside and go in. The water has more oxygen than the blood in the area of the gills so oxygen diffuses into the blood and attaches to hemoglobin where it is then carried to the rest of the body. When the O2 rich blood reaches areas of the body that need O2, such as areas of high metabolic activity like the muscles, the O2 moves off the hemoglobin and diffuses into the cell.

In these areas cells are using O2 and producing CO2 through metabolic processes. CO2 diffuses from high concentration areas around the cell into the lower concentration area in the blood where it attaches to the hemoglobin that has been vacated by the O2. This is not causative. The CO2 does not displace the O2 from the hemoglobin and vice versa. The CO2 is carried by the blood back to the gills where it diffuses out of the blood and into the water.

Now here is why this is relevant. Normally CO2 diffuses from the high concentration area in the blood at the gills to the low concentration area in the water. If the concentration of CO2 in the water is too high CO2 will not be able to diffuse as completely from the blood and the concentration of CO2 in the blood and therefore the tissues in the body will rise. If the concentration of CO2 in the water is higher than the concentration of CO2 in the blood at the gills it would actually flow in reverse, diffusing into the blood through the gills. This has nothing to do with and does not affect the movement of O2 which will still move into the blood in the gills as long as the concentration of O2 is higher in the water than in the blood. The problem is that CO2 is not being evacuated from the blood. This causes all sorts of problems that I dont want to go into here but if CO2 stays in the blood it will quickly overwhelm the bodies ability to compensate and the fish (or whatever organism) will die.

I believe that the reason fish are seen to "gasp" at the surface is two fold. First, the concentration of CO2 in the water would be lowest near the surface where it is outgassing. Second, the "gasping" would move more water over the gills to speed the process. I think it has nothing to do with the level of O2, but in fact is the fish trying to move CO2 out of its blood.

*edit*
Upon further thought I am not sure that my last sentence is always true. Gasping at the surface would not only move CO2 out of the blood but would move O2 into the blood for the same reasons, higher O2 concentation at the surface and moving more water to speed the process.

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post #11 of 33 (permalink) Old 01-10-2009, 09:23 AM
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Rod,

Grab a cup of Joe, and chew on this with some thought as to you're tank being a living, breathing
thing.

Biochemical Oxygen Demand, or BOD, is a measure of the quantity of oxygen consumed by
micro-organisms during the decomposition of organic matter. BOD is the most commonly used
parameter for determining the oxygen demand on the receiving water of a municipal or industrial
discharge. BOD can also be used to evaluate the efficiency of treatment processes, and is an
indirect measure of biodegradable organic compounds in water.

Imagine a leaf falling into a stream. The leaf, which is composed of organic matter, is readily
degraded by a variety of micro-organisms inhabiting the stream. Aerobic (oxygen requiring)
bacteria and fungi use oxygen as they break down the components of the leaf into simpler, more
stable end products such as carbon dioxide, water, phosphate and nitrate. As oxygen is
consumed by the organisms, the level of dissolved oxygen in the stream begins to decrease
water can hold only a limited supply of dissolved oxygen and it comes from only two sourcesdiffusion
from the atmosphere at the air/water interface, and as a byproduct of photosynthesis.
Photosynthetic organisms, such as plants and algae, produce oxygen when there is a sufficient
light source. During times of insufficient light, these same organisms consume oxygen. These
organisms are responsible for the diurnal (daily) cycle of dissolved oxygen levels in lakes and
streams.

If elevated levels of BOD lower the concentration of dissolved oxygen in a water body, there is a
potential for profound effects on the water body itself, and the resident aquatic life. When the
dissolved oxygen concentration falls below 5 milligrams per liter (mg/l), species intolerant of low
oxygen levels become stressed. The lower the oxygen concentration, the greater the stress.
Eventually, species sensitive to low dissolved oxygen levels are replaced by species that are
more tolerant of adverse conditions, significantly reducing the diversity of aquatic life in a given
body of water. If dissolved oxygen levels fall below 2 mg/l for more than even a few hours, fish
kills can result. At levels below 1 mg/l, anaerobic bacteria (which live in habitats devoid of
oxygen) replace the aerobic bacteria. As the anaerobic bacteria break down organic matter,
foul smelling hydrogen sulfide can be produced.

BOD is typically divided into two parts- carbonaceous oxygen demand and nitrogenous oxygen
demand. Carbonaceous biochemical oxygen demand (CBOD) is the result of the breakdown of
organic molecules such a cellulose and sugars into carbon dioxide and water. Nitrogenous
oxygen demand is the result of the breakdown of proteins. Proteins contain sugars linked to
nitrogen. After the nitrogen is "broken off" a sugar molecule, it is usually in the form of ammonia,
which is readily converted to nitrate in the environment. The conversion of ammonia to nitrate
requires more than four times the amount of oxygen as the conversion of an equal amount of
sugar to carbon dioxide and water.

When nutrients such as nitrate and phosphate are released into the water, growth of aquatic
plants is stimulated. Eventually, the increase in plant growth leads to an increase in plant decay
and a greater "swing" in the diurnal dissolved oxygen level. The result is an increase in microbial
populations, higher levels of BOD, and increased oxygen demand from the photosynthetic
organisms during the dark hours. This results in a reduction in dissolved oxygen concentrations,
especially during the early morning hours just before dawn.
In addition to natural sources of BOD, such as leaf fall from vegetation near the water's edge,
aquatic plants, and drainage from organically rich areas like swamps and bogs, there are also
anthropogenic (human) sources of organic matter. If these sources have identifiable points of
discharge, they are called point sources. The major point sources, which may contribute high
levels of BOD, include wastewater treatment facilities, pulp and paper mills, and meat and food
processing plants.

Organic matter also comes from sources that are not easily identifiable, known as nonpoint
sources. Typical nonpoint sources include agricultural runoff, urban runoff, and livestock
operations. Both point and nonpoint sources can contribute significantly to the oxygen demand in
a lake or stream if not properly regulated and controlled.

Performing the test for BOD requires significant time and commitment for preparation and
analysis. The entire process requires five days, with data collection and evaluation occurring on
the last day. Samples are initially seeded with microorganisms and saturated with oxygen (Some
samples, such as those from sanitary wastewater treatment plants, contain natural populations of
microorganisms and do not need to be seeded.). The sample is placed in an environment suitable
for bacterial growth (an incubator at 20o Celsius with no light source to eliminate the possibility of
photosynthesis). Conditions are designed so that oxygen will be consumed by the
microorganisms. Quality controls, standards and dilutions are also run to test for accuracy and
precision. The difference in initial DO readings (prior to incubation) and final DO readings (after 5
days of incubation) is used to determine the initial BOD concentration of the sample. This is
referred to as a BOD5 measurement. Similarly, carbonaceous biochemical oxygen test
performed using a 5-day incubation is referred to as a CBOD5 test.

Fish waste, excess food and even filter media play a vital role in the success or failure of this
environment. But ultimately it is the lack of oxygen that kills fauna, very high levels of C02 can do
this also obviously, but with high levels of C02 you also get lower levels of oxygen, so the proper
amount of surface agitation is vital.

High quality filter and media plays a vital role in this environment also because it helps control
nitrate, nitrite and ammonia..nitrate is also a form of acid, nitric acid.

It is no exaggeration to say that the condition of an aquarium depends very much on the performance
of its biological filter. When the filter's micro-organisms are thriving, the water will be crystal
clear and there is no algae growth.

The chemical reaction that expresses the oxidation process carried out by the nitrobacteria which
converts harmful ammonia into harmless nitrate is NH3; NO2; NO3. The bacteria that converts ammonia
(NH3) into nitrite (NO2) is called Nitrosomonas, and the bacteria that converts that into nitrate
(NO3) is called Nitrobactor. Research shown that the remaining nitrate is about 70 times less toxic
than nitrite, but if enough accumulates in the water it canstill be harmful. Therefore, it is always
necessary to frequently change the aquarium water even when using a top-of-the-line filter.

So we need good filters and media, lots of oxygen or surface agitation, but not to the degree that we
degrade our level of C02 content....which will be evident by algae growth, and or poor plant health based
on the amount of light over the tank, and keeping the tank clean.

Injecting to much C02 into the water to the point of choking fish you are in fact damaging the micro-
organisms at the same time, don't do that.

It is an environmental thing.

Craig

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post #12 of 33 (permalink) Old 01-11-2009, 01:22 AM
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blood more readily obsorbs co2 than o2 so if concentrations are too high more co2 is exchanged in the blood and not o2.... its not b/c of displacment its because a flux in the concentration between the 2 gases in the water.... if i pumped co2 in a room a simmilar thing would happen not because o2 is magically dissapearing but because you will absorbe more co2
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post #13 of 33 (permalink) Old 01-11-2009, 02:33 AM
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Quote:
Originally Posted by imeridian View Post
I suggest some reading on the subject of hypercapnia.
Very interesting! So I'm taking the scuba gear away from my fish.

I'm not a biologist, botanist, nor chemist and not even a MD, so my understanding of this has some slim chance of being wrong. Lungs and gills work to expel the waste product of breathing, which is CO2, by concentrating it in the blood to a level higher than that in the surroundings, air or water, then exposing it to the surroundings so the CO2 in the blood diffuses into the air or water. But, if the amount in the air, for us land dwellers, or in the water, for fish, is too high, neither of us can concentrate it in our blood to a high enough level to be able to get rid of it, before that level is too high for us to tolerate. Thus, hypercapnia! Those fish "gasping" at the surface are in the water with the lowest concentration of CO2, that near the surface, and are trying to reach up even higher to find a still lower concentration. The fish that can't or don't go to the surface and stay there, like my loaches, just lay on the substrate and turn pale, like I would in a similar situation.

Now, aren't you glad one of us is so under educated that this is easy to understand?

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post #14 of 33 (permalink) Old 01-11-2009, 03:54 AM Thread Starter
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This has been a great thread and I really appreciate all the thought people have put into their answers. I learned a lot. As for gasping at the surface, I think Airborne R6 may be on to the heart of the problem. A biologist can tell me if I am right or wrong about this, but I think that blood uses the same mechanism to carry O2 and CO2, so a fish's bloodstream has a finite capacity to carry those two gases. Henry's law does not apply to the hemoglobin in the bloodstream. So, if the concentration of CO2 in the water is high and that means the fish cannot outgas CO2, it also would mean that it cannot use that hemoglobin to pick up oxygen in the gills.

By gasping at the surface, the fish may be able to outgas the CO2, since the concentration of CO2 above the water surface is not as great as the concentration within the water.

Anybody want to second that?

Thanks,
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post #15 of 33 (permalink) Old 01-11-2009, 05:17 AM
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Hi WfxXx

Reading your interesting posts are hard for me to follow because your page is just a tad over 1/2 of the normal page width. My eyeballs have to travel back and forth almost twice as many times. It is hard for me to follow what you are actually saying. With the small dwell time that it takes for my eyeballs to get back to left side again, I've forgotten much of what I just read. My eyes feel like they are on a typewriter carriage going back and forth. I guess that I am just getting old.

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