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CO2 Poisoning


  1. CO2: How much is too much?
    by (Grant Gussie) (20 Nov 93)
  2. Is CO2 Safe for Men?
    by George Booth
  3. Bubbling plants
    by (Paul Nicholson) (Tue, 25 Jun 1996)
  4. re: Low oxygen vs. high CO2
    by Mark Fisher <Mark.Fisher/> (Wed, 30 Sep 1998)
  5. CO2 reg. goes insane, Help!
    by (Sperry Sperry quite contrary how do your batch jobs CO2 reg. goes insane, Help! (22 Aug 94)
  6. Yeast Explosion
    by "David Huie" <David.Huie-at-Bentley.COM> (Mon, 17 Jul 95)
  7. Re:CO2 and Science
    by "Roger S. Miller" <rgrmill/> (Sat, 22 Jan 2000)
  8. CO2 transport in FISH, as well as us lowly humans.
    by Alec Dale <dale7/> (Sun, 16 Jan 2000)

CO2: How much is too much?

by (Grant Gussie)
Date: 20 Nov 93

In article <>, (Dieter Kreuer) wrote:
> In article <2br2qj$>, (Kaufman M.E.) wrote:
> Very much CO2 surely is good for the plants, but if pH
> is 6.6 and stable, is that too much for the fish? It didn't look like
> that to me.
The reason excessive CO2 is harmful is because it causes a condition called
blood acidosis, where the blood pH drops below its healthy alkaline range.
Assuming the fish can handle a water pH of 6.6 (almost all freshwater fish
can) since the blood pH of a fish is linked to its environment by the
passage of ions across the gill membranes I doubt that the blood pH would
be dangerously low at that CO2 concentration.

Is CO2 Injection Really Safe for Men?

by George Booth

> Is there a (cheap) method to measure air CO2 %age? Or (better) a
> CO2-watching alarm? Or (best) is my CO2 fear simply stupid & unjustified?

The CO2 level in your tank will be about 15-20 ppm.  The CO2 level
in the atmosphere is ~350 ppm.  Unless you are in an absolutely, 
completely airtight room, the CO2 diffusing from your tank will
be insignificant.  Even in an airtight room, it would take *quite*
a while for the CO2 to build up to problem levels.  You would probably
die of radon poisoning first. 


Bubbling plants

by (Paul Nicholson)
Date: Tue, 25 Jun 1996

George Booth repled to Jim Kostich:

>> Another possibility along this line of thought is that the water might
>> well continue dissolving O2, but be dumping some other gas
>> simultaneously to "make room".
>No, each gas has its own soluability (partial pressure).  One type
>will not leave to make room for another.
>> The above is all based on my assumption that the maximum solubility of
>> each of the gases involved is limited by the presence of other gases
>> already dissolved - an assumption which makes sense to me,
>That is incorrect.  Many fish stores continue to advise against CO2
>injection "because it will reduce the oxygen available to the fish".
>It just doesn't work that way.

I disagree with this analysis. Addition of any gas will lower the partial
pressures of other gases. A liquid in equlibrium with a mixed gas
atmosphere can hold exactly 1 atmosphere of mixed gas. If we increase the
amount of one gas by a diffusion scheme like our CO2 "reactors" we increase
its partial pressure, therefore the total partial pressures of the other
gasses will be reduced.

Imagine an aquarium with half of the surface exposed to air and half
exposed to CO2. In this case the average atmosphere above this tank is 50
percent CO2, 10 percent O2, and 40 percent N2. Obviously the partial
pressures of N2 and O2 will be reduced.

In this case since a continuous transport of the the gases from one
atmosphere to the other will be established the partial pressures in the
liquid will also be affected by how efficiently each gas diffuses
into/outof the liquid, I don't know if different gases have different
diffusion rates into a liquid or not, however I suspect so, since this is
"sorta" how a gas chromatograph works.

Anyway not to worry, the partial pressures of CO2 that we are using are so
low that not much O2 or N2 leaves.


re: Low oxygen vs. high CO2

by Mark Fisher <Mark.Fisher/>
Date: Wed, 30 Sep 1998

>I think 2.8ppm is pretty low and it showed in  _very_ rapid breathing
>the SAEs ... but they still lived.  So I pose the question:  is it
>in the ABSOLUTE level of oxygen which bothers the fish or is it a
>RELATIVE to the level their accustomed level?

It's a reduction in the absolute level of oxygen.  For fish, the %
saturation curve of oxygen with hemoglobin is dependent upon the amount
of oxygen dissolved in the water.  This graph, if it makes it through,
shows the % saturation of oxygen with hemoglobin as a function of the
amount of oxygen dissolved in water:

    100|            *  * * * * *  X ppm CO2    
       |          *           
       |        *    * * * * * *  2X ppm CO2        
       |       *  *             
 %  50 |     *  *          
       |    *  *            
       |   *  *             
       |  *  *                 
     0 |__*_*_____________________
       0          5            10
			ppm O2

CO2 can reduce the oxygen affinity of hemoglobin, which is why I drew
two curves.  The top curve shows the percent oxygen saturation at an
arbitray level of CO2, X.  Note 100% saturation occurs at about 6 ppm
oxygen, and drops off sharply below 6ppm.  Death would be expected to
occur below about 3 ppm oxygen.  At twice the level of CO2 (lower
curve), 100% saturation is never reached because of the reduced affinity
for oxygen.  Instead, about 75% saturation is the maximum at about 6
ppm, and drops off sharply below that.  This phenomenon is known as the
root effect.  Death would be expected to occur at a slightly higher
level of oxygen than the upper curve.  Also, if the peak activity of a
fish requires 80% or higher oxygen saturation, then they could go
anaerobic pretty quickly, and crash.

Note that different species of fish have different saturation curves.
Some fish have relatively flat curves, and need high levels of oxygen to
achieve maximum saturation (e.g., rainbow trout), while others have a
fairly steep curve and can achieve maximum saturation at much lower
oxygen levels (e.g., catfish).  The presence of CO2 affects them all,
and shifts their curves to the right, requiring higher levels of oxygen
to achieve the same % saturation.  At high CO2, 100% saturation never
occurs, and very high levels of CO2 can block the uptake of oxygen
altogether, regardless of how much is in the water.  Fish can suffocate,
even in oxygen-saturated water, if CO2 is high enough.

Any blood chemistry or animal physiology reference covers this is much
greater detail.  15 ppm CO2 is a safe level for our finny friends, but
definitely exacerbates low oxygen problems.  I suspect SAE's have a
fairly flat saturation curve, and require relatively high levels of
dissolved oxygen to achieve maximum saturation, and/or they may be more
sensitive to CO2 than most fish.



CO2 reg. goes insane, Help!

by (Sperry Sperry quite contrary how do your batch jobs CO2 reg. goes insane, Help!
Date: 22 Aug 94
Newsgroup: sci.aquaria

CO2 accident
About 2 days ago out regulator on our CO2 regulator went nuts and dumped the
whole tank of CO2 (about 500 pounds, the tank had only been used for a few
days) into our 29 gal. fish tank.  Unfort. when this happened we were gone for
the day and didn't return till about 4 or 5 hours later.  I know it didn't
start before we left because I tested the water's ph and finding it at (down .5
from the prev. night and I wanted to keep it at 7) 6.5 I turned down the flow
before we left (and I am pos. I turned it down).  I am sorry now that I didn't
turn it off, which I prob. should have done.  We had been noticing that the
regulator was sliping a little, but no where near that much.  

Anyway needless to say most of the fish were dead and I don't even know what
the ph was in the tank except that it was below 5 (My test kit only goes to 5). 
I quickly filled a bucket and threw all the fish in reg. tap water which is 8.5
(all I had available) (the reason for the CO2 along with plants).  I knew that
this would shock them too, but I couldn't think of anything else to do.  I
hoped it might bring them out of shock if they were still alive.  It seemed to

Dead Fish
One large angel  -  Dead
One med. juv. angel  -  Dead
One med. black shark  -  Dead
Three horsehead loaches  -  Dead
Two tiny Geo. Jur.  -  dead
Five or Six dif. types of cory cat  -  Dead (one lived for a day)
Four 2 tiny, 2 med, clown loaches  -  Dead (I had had the largest one since I
					    got into fish again about 5 or 6
					    years ago)
Three lemon tetra's, small  -  Dead
Three black tetra's, large  -  Dead
One hatchet fish, large  -  Dead

Fish living this morning
Five kui type loaches  -  so far all are alive
One small upside down cat  -  so far alive (seemed dead, but came back to life)
One large green cory  -  so far alive
One small bumble bee cat  -  so far alive
One small ? cat  -  so far alive

Yes we did have a lot of filtration on the tank!

What I want to know is can I expect the rest of the fish to live since they
have survived for a couple of day's?  Is there anything I can do that might
help them becides just getting them back in normal water conditions?

They seem to be doing ok, they eat and everything.  The upside down cat had
some prob. swimming at first, but he is fine now.

We are going to try to get a medical regulator for the CO2, so this doesn't
happen again.

Trice L. Hundstad               | Mankato State University      | Mankato, MN USA
Scamper: "AL" Bd+S+O G 00.07 X++|  >*}  0> ^..^ /^==
(& SAM)  L- W C+++ I+++ T+ A+   | Fish+Bird+Cat+Horse=Way To Much Work!
         E+++ H S++ V+++ F++ Q P| They are worth it though! :^)
         B+ PA+ PL++            | BTW:  All spelling errors are due to S&H.

Yeast Explosion

by "David Huie" <David.Huie-at-Bentley.COM>
Date: Mon, 17 Jul 95

     I've had all the fish in my tank killed by acidosis...twice now.  Both 
     times were as a result of massive pH changes due to my inaccurate CO2 
     injection method.
     The first time, the tank just ran out and the pH went from 6.2 to 7.8 
     in about two hours.
     The second time, last week the whole CO2 tank vented into the aquarium 
     at once--a one inch shell of ice on the CO2 canister was a bit 

Re:CO2 and Science

by "Roger S. Miller" <rgrmill/>
Date: Sat, 22 Jan 2000

Sylvia wrote:

>I believe the thread on CO2 and its effects on fish got completely out of
>hand and irrelevent to tank situations. Technical discussions were centered
>around the knowledge or expertise of the member . . . rather than practical
>discussions on the effect in the tank, and on fish. Some people walked away
>with the understanding that CO2 was not harmful in their tank (with the 
>implied suggestion that it did not need to be so closely monitored) with the
>net result that fish are are now dying. Someone even responded to one post,
>suggesting that it wasn't the CO2 in the tank,  but instead detergent or
>something else that had gotten into the tank, that killed the fish.


Thank's for your well-written post.  It appears that you and I got very
different messages from the discussion.

What I understood is that high CO2 levels may cause some inhibition of
respiration in fish, but it's unlikely that high CO2 levels in the water
would lead directly to symptoms of suffocation.  Acidosis -- both
environmental due to low pH in the water and respiratory due to
accumulation of CO2 in the fish -- was mentioned as a direct effect from
high CO2 in the water.

Also, the point was made very clear that CO2 accumulated *above* the water
can cause asphyxiation by preventing atmospheric oxygen from entering the
water.  The same effect could be obtained from a layer of nitrogen or
argon (for instance) above the water.  It isn't a specific effect of CO2.

In practical terms, that means that if you turned up the CO2 input to your
tank and you found fish gasping at the surface in the morning then you
need to get the top off your tank and get them some fresh air.  Second,
you need to temporarily increase aeration or circulation until the fish
return to normal.  After that you can look to the cause of the problem.  
It may be caused by a number of things - detergent, for instance.  If it
is caused by CO2 then make sure that you can keep the air above the tank
from accumulating CO2 and maybe reduce the CO2 input.  Just turning the
CO2 down or off is unlikely to be the best or quickest route to solving
the problem and it may be completely useless.

For what it's worth, back when I started using CO2 I took the tight glass
tops off my tanks and either left them off or replaced them with lighting
eggcrate cut to fit the top of the tank.  That should prevent CO2 buildup
above the water.

>I believe science is very helpful in keeping fish and planted aquaria,
>obviously. But equally as important, or moreso, is common sense, respect for
>life and keen powers of observation. Unfortunately, as I said, when people
>read these highly technical discussions, they may ignore what common sense
>tells them in deference to the opinions of those that appear more intelligent
>and knowledgeable than themselves.

I believe that science is useless unless the technology is explained so
that people who need to use it can understand it.  Even with a good
explanation inaccurate data from hobby kits or a failure to fully grasp
the context of an idea can quickly turn "good science" into a big waste of
time and money or worse, lead to the death of animals and plants.

Those of us who do get into the technical arguments need to be more
careful to summarize what the arguments mean in non-technical terms.  
That is one of the most difficult things for a technical writer to do.

Roger Miller

CO2 transport in FISH, as well as us lowly humans.

by Alec Dale <dale7/>
Date: Sun, 16 Jan 2000

> >Date: Fri, 14 Jan 2000 09:45:11 -0500
> >From: "Peter G. Aitken" <>
> >
> >As I detail in another post, CO2 does NOT bind to hemoglobin.
> ...
> >Date: Fri, 14 Jan 2000 09:46:24 -0800
> >From: Dave Gomberg <>
> >
> >At 03:48 AM 1/14/2000 -0500, Sherman wrote:
> >>Someone said recently that CO2 was transported out of the system by
> >>hemoglobin.
> >
> >I believe this is bushwa (at least in humans).
> ...
> >From: "Booth, Karla" <>
> >Date: Fri, 14 Jan 2000 17:18:26 -0700
> >
> >The comment is wrong because hemoglobin does carry CO2 - that is how
> >CO2 is transported from your body, a natural waste from metabolism.  Your
> >body is always handling CO2 and O2 transport and the amount transported is
> >dependent on the concentration of O2, hydrogen ion concentration (pH),
> >concentration of 2,3 DPG, and concentration of CO2 at the site.  In your
> >lungs with high O2 a release of CO2 is favored and O2 is bound - in
> >exercising muscles, veins, etc where the concentration of CO2 is high, pH is
> >low (and I think 2,3 DPG is high) a release of O2 occurs and CO2 is bound.
> >It is a very complicated mechanism - not just a high concentration of one
> >gas causes that gas to be bound and carried by hemoglobin.
> >
> >Carbon monoxide is so dangerous because it is irreversibly bound by
> >hemoglobin.  Once it binds to Hb, that molecule no longer can transport
> >oxygen so with continuing CO input - you die.
> Since Karla has a PhD in Biochemistry (among other reasons :-), I'll go
> with her statement.
> George Booth, Ft. Collins, Colorado

O.K. I am going to chime in on this one, since there are lots of educated
people out there all giving bits of the correct story, but no one seems to be
referring to FISH physiology in any of their posts.  Additionally, some people seem
to be answering off the top of their head.  That's great if you can do it,
but I'm a lowly fish ecologist, so I will try to cite my information where I can.
A good portion of this info. is from Hoar (1983) and Evans (1993).  Note, if you
don't want to
hear about the physiology of CO2 transport....skip this LONG post.

First, for vertebrates in general, CO2 transport is carried out by 3 mechanisms.
Approx., 81% is through several different blood buffer systems,
11% is through carbamino compounds (including but not limited
to hemoglobin), and the remaining 8% is carried in simple solution
(Hoar, 1983)(note these are averaged values).  Since CO2 plays such
a major role in the blood buffer system, its transport and acid base
regulation are inseparable and too extensive to cover in a
post, even on APD ;-) .  I will try to summarize the actual transport
and exchange.

In mammals, carbamino compounds carry a relatively small % of the
blood CO2; however, they play a key role in respiration.
This is primarily due to the fact that the amino (NH2) groups in Hb
(as well as other blood proteins) link with CO2 in a
"non-enzymatic reaction".  This is important since this reaction is so
rapid and because carbaminohemoglobin (use that in Scrabble) forms
a reversible reaction with O2:
               O2 + HHbCO2 <---> HHbO2 + CO2
This reaction allows CO2 to be rapidly taken up or released
without significant changes in blood pH (i.e. without affecting
the blood buffer system).

In the body the deoxyhemoglobin is relatively alkaline and combines
with increasing amounts of carbonic acid (which is increasing...refer
to George/Karla's post).  See, it's all intertwined with pH regulation ;-).
In the lungs the mechanisms are reversed and CO2 is readily
discharged (Hoar, 1983).

Additionally, since hemoglobin (Hb) is a potassium salt it also acts as one
of the major blood buffers by combining with carbonic acid in the following
            KHb + H2CO3 <--> KHCO3 + HHb

Now the problem with comparing fish to humans or other mammals
in terms of CO2 transport, really makes the apples and oranges
analogy a little too tame, more like apples and carrots.
The reason for this is that respiration and pH regulation are obviously
very different.  Aquatic animals have very low blood levels of CO2
(bicarbonate) compared to terrestrial animals.  The key reason is that fish
must circulate large volumes of water over their gills to get O2, and water
dissolves CO2 about 200 times more rapidly than O2.  Thus, CO2 diffuses
rapidly into the water via the gills so that there is only a small difference
between the levels of CO2 in arterial blood and the surrounding water (Hoar, 1983).

In fish, CO2 transport is also carried out via the 3 mechanisms I mentioned
above; however, the ratios and importance in gas exchange of each are different.
Physically dissolved CO2 usually constitutes <5% of the total CO2 in the blood of
CarbaminoCO2 (including HHbCO2) is relatively unimportant in fish
(Perry and McDonald, 1993).  Perry and McDonald (1993) speculate that it is due to
the acetylation of the terminal amino (NH2) groups on the alpha chains of hemoglobin

(take from that what you will).  The largest fraction (90-95%) of total CO2 in the
of fish exists as HCO3- .  HCO3- is carried both in the red blood cells (rbc), and
blood plasma (majority).

The general mechanism is as follows [summarized from Evans (1993)]:
CO2 diffuses from the tissues into the plasma.  During venous transit, the
CO2 is converted to HCO3-  via catalyzation by carbonic anhydrase
(enzyme) within the rbc.  This HCO3- exits the rbc in exchange for plasma
Cl- (chloride shift, and I'm not getting into that either).  As blood flows through
the gills, HCO3- again enters the rbc in exchange for intracellular Cl-, and is then

dehydrated to CO2 with carbonic anhydrase.  This CO2 then diffuses into
the plasma and then into the water via the gill epithelium.  The excretion
of CO2 is largely a simple diffusion mechanism; thus, CO2 content
of the blood is similar to the surrounding water.

So people are correct in saying that hemoglobin plays a direct and significant
role in CO2 transport in mammals, but NOT when referring to fish.  Granted,
Hb still plays a key role since H+ ions for the dehydration of HCO3- in the
rbc are supplied by buffer groups on hemoglobin during oxygenation
(but I'll spare you that detail as well ;-).

Hope this helps a bit.  You can deduce from this that increased CO2
in the water will not directly prevent hemoglobin in fish from carrying O2, since
Hb does not play a significant role in CO2 transport in fish.  However, at
high enough levels CO2 in the water will reduce the diffusion of CO2 from
the blood, resulting in acidosis among a long list of other consequences.  I do not
know what behaviour this would illicit, but I do not find it impossible that
the fish would increase respiration, not to obtain more O2, but to get rid
of excess CO2.  Since CO2 is so soluble in water, It makes sense that
it would take quite a high concentration to impede diffusion from the gills.

That's the way I see it!  If this lowly ecologist has interpreted anything
incorrectly (quite probable) feel free to post corrections or [like you all wouldn't
;-) ]

Literature Cited

Evans, D.H., ed. 1993.  The Physiology of Fishes.  CRC Press,
     Boca Raton, Ann Arbor, London, Tokyo: 592pp.

Hoar, W.S. 1983. General and Comparative Physiology. Third Edition
    Prentice-Hall, Inc., Englewood Cliffs, New Jersey: 851pp

Perry, S.F., and G. McDonald.  1993.  Gas Exchange: 251-278. In
    Evans, D.H., ed. 1993.  (see above).

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