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Re: CO2, gasping, etc

I agree with the majority of what George wrote here, but want to add my
two bits.
On Wed, 12 Jan 2000, George Booth wrote:

> In our one tank with manual CO2 injection running continuously, we
> would see O2 saturation vary from around 120% in the evening to 90% in
> the morning before the lights came on. Given that you can generally
> only achieve 95% O2 saturation even with the best airstones running
> full blast, going "down" to 90% at night isn't all that much of a
> drop.

I can confirm this observation from a tank without CO2 injection.  It's
been a few years since I measured these values, but my recollection of it
is pretty clear because I was surprised by the results.  In a planted tank
with a moderate fish load and good circulation (and using a high quality
O2 kit), I found 10 mg/l O2 near the end of the day and 8 mg/l just before
lights went on in the morning.  I calculated that saturation for my
altitude and tank conditions was something like 8.4 mg/l.

That is part of my reason for saying that gasping at the surface in the
morning would probably reflect over crowding, overfeeding or insufficient
circulation.  In the absence of those problems, the oxygen content of the
water should stay with the fish's confort zone.


> Your fish are going to get stressed only if CO2 increases TOO much
> overnight or the O2 is reduced TOO much overnight. Even with high
> oxygen levels, fish cannot respire CO2 unless the concentration of CO2
> in the water is less than the concentration of CO2 in the fishes
> blood. The exchange of O2 and CO2 in the gills is determined by the
> relative concentration of each gas on both sides of the gill membrane
> and the exchange of each gas at the membrane is independent of the
> other.
> Many people miss this point. With high O2 AND high CO2 in the water,
> CO2 cannot leave the blood (meaning O2 can't enter the blood; they
> bind to the same site on the hemoglobin molecule) and the animal
> asphyxiates. With low O2 AND low CO2 in the water, CO2 can leave the
> blood but O2 can't enter and the animal asphyxiates.

Feroze Ghadially (in Advanced Aquarist Guide) wrote a discussion of CO2
excess and O2 deficiency symptoms.  He suggested first that the
suppression of oxygen uptake by an excess of CO2 only effects a few
species.  Further he wrote (page 21):

   "In an experimentally created situation where there is a lack of oxygen
   without carbon dioxide excess, fishes come to the surface and behave as
   they do in crowded aquaria, but when there is an excess of carbon
   dioxide but no oxygen lack they may not behave in this fashion.  In
   fact they appear drowsy, rest on the bottom and rock from side to

Dr. Ghadially's book is ancient (copywrite 1969 - about the time that my
little sister turned my first aquarium into a hamster home) but he was
very careful with his data and there's little that he found to be true in
1969 that isn't still true today.  Besides, I'm reasonably sure that I've
read elsewhere that the symptoms of CO2 excess are distinctly different
from the symptoms of oxygen deprivation.  I just couldn't find that
reference :).


> Also consider the dynamics of CO2 usuage. The are two ways CO2 is
> removed from the water: dissipation into the atmoshere and usage by
> the plants. I have not directly measured this, but I conjecture that
> the amount used by plants is far less than the amount lost to the
> atmosphere.  This means that the concentration in the water during a
> 24 hour cycle only changes by the amount the plants are actually
> using. The concentration goes down a little during the day and goes up
> a little at night, causing a far smaller pH change than if you turned
> CO2 off at night (a difference of 0.3 in our case).

Several people observed fairly small day-night pH swings in their tanks
with continuous CO2 injection.  I agree that this is probably evidence
that dissipation to the atmosphere is usually the main factor controlling
CO2 concentrations.  Plant use would have a secondary effect. It seems to
me that dissipation to the atmosphere should prevent dangerously high CO2
concentrations from building up at night.  That assumes that the daytime
CO2 concentrations are far enough below toxic levels to provide a margin
of safety.

And George, I'm glad to read you on the list again.

Roger Miller