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Leave CO2 On At Night?

Contents:

  1. CO2, gasping, etc
    by George Booth <booth/frii.com> (Wed, 12 Jan 2000)
  2. CO2, gasping, etc
    by "Roger S. Miller" <rgrmill/rt66.com> (Wed, 12 Jan 2000)
  3. Aquatic Plants Digest V4 #26
    by Rebecca Allbritton <rda3507/acs.tamu.edu> (Thu, 13 Jan 2000)
  4. CO2, gasping
    by George Booth <booth/frii.com> (Thu, 13 Jan 2000)
  5. CO2 system with/without pH controller
    by "Peter G. Aitken" <peter/pgacon.com> (Fri, 14 Jan 2000)
  6. CO2 system with/without pH controller
    by Dan Dixon <dandixon/home.com> (Fri, 14 Jan 2000)
  7. CO2 and O2 - a clarification
    by "Peter G. Aitken" <peter/pgacon.com> (Sat, 15 Jan 2000)
  8. Controller/Doser/Evaporate
    by Jared Weinberger <jweinberger/knology.net> (Sat, 27 May 2000)
  9. Aquatic Plants Digest V4 #222
    by "II, Thomas Barr" <tcbiii/earthlink.net> (Sun, 16 Apr 2000)

CO2, gasping, etc

by George Booth <booth/frii.com>
Date: Wed, 12 Jan 2000

There have been a plethora of postings recently that demonstrate a pretty
clear misunderstanding of CO2, respiratory systems, O2<->CO2 relationships
and KH/pH/CO2 relationships. I had a private e-mail recently about such
things and I thought I would post my answer, just to add to the fray.

I wrote:
>> "A manual setup works fine but takes constant fiddling to balance it.
>> The CO2 bubbles in continuously (DON'T shut it off at night) and you

You write: 
> By the "(DON'T shut it off at night)" are you just describing what it
> does, or saying that you don't want to stop this at night.  

I'm saying you do NOT want to turn off the CO2 at night, IMHO. 

> I guess I am asking the basic
> question of - does [using a solenoid to turn off CO2 at night] sound
    
> reasonable?  

No, IMHO. 

> If I run it
> continuously, my pH will be lower at night than during the day when the
> plants produce O2.  

Better stated as "The pH will be lower at night with the CO2 on because the 
plants are not photosynthesizing and using their share of CO2." 

> However, if I run against the timer, the pH will
> probably increase during the night due to the lack of CO2.  

It will *definitely* increase at night. In a reasonable tank set up with
proper 
water circulation, ALL the dissolved CO2 will dissipate into the atmosphere. 
Check out my article "CO2 loss in a large aquarium" on my web page or the
krib 
archives.   

Take an example. Consider a typical setup with proper CO2 injection.  A 
reasonable set of parameters is 5 dKH and 15 mg/l of dissolved CO2
establishing 
a pH of 7.0. With the CO2 turned off overnight, dissovled CO2 will be
reduced to a typical equilibrium value of 1.5 mg/l, creating a pH of 8.0!
That's quite a pH swing daily. I would not consider this a good environment
for fish.

> I like the
> idea of the timer for a few reasons.  First, it gives the fish a break
> during the night when O2 will be lower because the plants are not
> contributing; 

With proper water circulation, O2 levels will not drop appreciably overnight. 
Although I've seen reports on APD where someone's fish were distressed, 
presumably by a lack of O2, I've never seen this confirmed by anyone who paid 
the least bit of attention to what was going on. I would attribute these
reports to a newbie hacking around with CO2 and water chemistry and water
movement (creating generally horrible conditions in the tank) and then
blaming the problems on CO2 injection.  

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. 

Remember, there is no relation between the amount of O2 and the amount of
CO2 in the water (within reason). The O2 level in the water is determined
by diffusion at the water surface, proper circulation (mixing surface water
and lower water), the amount used by the aquarium bio-life and the amount
contributed by photosynthesis. Turning off CO2 at night does nothing to
prevent O2 from dropping. O2 is going to drop when the lights go out since
the plants stop photosynthesizing.  
 
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. 

> Second, it should save CO2.  

The amount of CO2 you save is not really worth considering when you figure
what 
CO2 costs (assuming pressurized CO2 bottles). I would venture a guess that
the 
cost of the solenoid will be far greater than what you would save in CO2
costs 
over many years of operation. 

Also, consider the dynamics of the situation. Assume we start at a steady
state 
CO2 level in the evening, save 15 mg/l.  The lights are on, plants are
bubbling, O2 is high, everybody is happy.

Now the lights go off. CO2 stops and the dissolved CO2 starts dissipating.
Since the level in the water is far above equilibrium values, the
dissipates rapidly at first, causing a fairly quick rise in pH (maybe 0.5
points over two hours). O2 also starts to dissipate. However, fish and
plants are going to sleep, presumably requiring less O2 as they go into a
resting state. 

Now it is morning and the lights come on. The fish and plants wake up, 
increasing the demand for O2. The CO2 comes on, but wait! There is very
little 
CO2 in the water and, since we aren't using an efficient CO2 reactor (since 
there is no control over how fast CO2 dissolves except for the "slow bubble 
rate"), it takes maybe hours for the CO2 level to reach the threshold where
the 
plants can begin synthesizing. So, for the first period of time, everything
is 
demanding more O2 but the plants aren't yet contributing any.

On the other hand, if the CO2 was running all night, there is now a little 
higher concentration of CO2 than in the evening and the plants can 
photosynthesize like gang-busters shortly after the lights come on. 

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).   

> I would really appreciate any
> thoughts you might have on the subject.

Hey, you hit me at the right time - plenty of thoughts AND some free time to 
write them down :-). 


 
George Booth, Ft. Collins, Colorado 


CO2, gasping, etc

by "Roger S. Miller" <rgrmill/rt66.com>
Date: Wed, 12 Jan 2000

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.

[snip]

> 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
   side."

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 :).

[snip]

> 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


Aquatic Plants Digest V4 #26

by Rebecca Allbritton <rda3507/acs.tamu.edu>
Date: Thu, 13 Jan 2000

>On Wed, 12 Jan 2000, George Booth wrote:
>
>> 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.

No, no, no -- you've forgotten about active transport, which is one way an
organism achieves higher concentrations of the desired material (some
gasses, e.g. O2, & some other molecules) inside the cell than outside, and
lower concentrations of unwanted materials inside than outside. 


back to lurking,
Rebecca
Genetics Major
Texas A&M University


CO2, gasping

by George Booth <booth/frii.com>
Date: Thu, 13 Jan 2000

>Date: Thu, 13 Jan 2000 12:39:27 -0600
>From: Rebecca Allbritton <rda3507@acs.tamu.edu
>
>>On Wed, 12 Jan 2000, George Booth wrote:
>>
>>> 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.

Karla (my biochemist wife) pointed out that I was oversimplifying this
process. CO2 and O2 have different affinity constants so there is some
complication with the actual concentrations and other details. But the
concept was OK.

>No, no, no -- you've forgotten about active transport, which is one way an
>organism achieves higher concentrations of the desired material (some
>gasses, e.g. O2, & some other molecules) inside the cell than outside, and
>lower concentrations of unwanted materials inside than outside. 

Karla said that active transport wasn't a factor in this. I'm taking about
gas exchange in the gills, not gas exchange at the cell level.
Unfortunately, she didn't elaborate. 


George Booth, Ft. Collins, Colorado


CO2 system with/without pH controller

by "Peter G. Aitken" <peter/pgacon.com>
Date: Fri, 14 Jan 2000

> C'mon, man, it's a scientific fact that O2 and CO2 are independent.
> Chuck Gadd's experiment demonstrated this. You are wrong. Period.
> Adding CO2 does NOT reduce O2.

I am afraid it is you who are wrong. Gasses in solution are not independent
of each other. Adding CO2 (or any other gas) to a tank will in fact reduce
O2. This is expressed by Henry's law which states that the volume of a gas
dissolved is proportional to its partial pressure. Chuck's experiments must
have overlooked something, or more likely the changes were not detectable by
his measurement methods. Gasses in solution are measured in terms of their
partial pressure, and at equilibirium the partial pressure of the dissolved
gasses will equal the pressure of the gas. For example, air is (appx) 78.97%
nitrogen, 21% oxygen, and 0.03% CO2, and at sea level the atmospheric
pressure is 760 mm Hg (millimeters of mercury, a measure of pressure also
called torr). At equilibrium, water will contain these gasses in the
following levels:

N2 600.17 mm Hg
O2 159.6 mm Hg
CO2 .22 mm Hg

The amount of each gas that is dissolved in the water will depend on the
temperature, but the proportion of the gasses depends only on their relative
partial pressures. Suppose then you add some CO2 to the system so that the
air now contains 77% N2, 18% O2, and 5% CO2. The total pressure remains the
same. The water will now contain the following:

N2 585.2 mm Hg
O2 136.8 mm Hg
CO2 38.0 mm Hg

By increasing the amount of CO2 from .03 to 5% you have decreased the amount
of O2 by 15%. Adding CO2 to a tank does reduce O2, despite the proclamations
of some "experts." That being said, the important question is whether within
normal paramters for an aquarium the addition of CO2 can cause enough of an
O2 drop to affect the fishes. I do not know the answer to this one, but it
is a possibility.

FWIW, I am a professor of physiology and I do in fact understand these
things!

Peter G. Aitken


CO2 system with/without pH controller

by Dan Dixon <dandixon/home.com>
Date: Fri, 14 Jan 2000

Peter G. Aitken at peter@pgacon.com wrote:

>> C'mon, man, it's a scientific fact that O2 and CO2 are independent.
>> Chuck Gadd's experiment demonstrated this. You are wrong. Period.
>> Adding CO2 does NOT reduce O2.
> 
> I am afraid it is you who are wrong. Gasses in solution are not independent
> of each other. Adding CO2 (or any other gas) to a tank will in fact reduce
> O2. This is expressed by Henry's law which states that the volume of a gas
> dissolved is proportional to its partial pressure.

I believe George was referring to the relationship between O2 and CO2 when
both are *in solution*. Henry's law pertains to the relationship of a
dissolved gas in proportion to its pressure in a gaseous state. In fact,
Henry's Law actually proves that George is right, since the addition of CO2
to water does not change the concentration of O2 in solution.

> At equilibrium, water will contain these gasses in the
> following levels:
[snip]
> Suppose then you add some CO2 to the system so that the
> air now contains 77% N2, 18% O2, and 5% CO2. The total pressure remains the
> same. The water will now contain the following:
> 
> N2 585.2 mm Hg
> O2 136.8 mm Hg
> CO2 38.0 mm Hg

Since CO2 is being added to the water via a pressurized tank, the partial
pressure in the atmosphere has little to do with it. Unless you change the
partial pressure of the O2 in the air above the water (which isn't gonna
happen in someone's house where their aquarium is), its concentration in the
water will not be affected by the addition of CO2 via a pressurized tank.

Dan Dixon


CO2 and O2 - a clarification

by "Peter G. Aitken" <peter/pgacon.com>
Date: Sat, 15 Jan 2000

I realize my earlier posts on gasses dissolved in tank water were not all
that clear, and if some people got confused I can see why! Let me try to
summarize in a way that is relevant for the aquarist.

1. Dissolving more of one gas in water decreases the amounts of other gas
(or gasses) that are dissolved. This was the one point I was trying to get
across in my original post; it is a long established principle and really
does not warrant debate.

2. In an aquarium, the amounts of CO2 that are injected will have at worst a
very minor effect on the O2 level. George Booth did the calculations and the
worst case scenario seems to be a 3% decrease in O2 if CO2 is injected to
optimal levels. This decrease in O2 is insignificant and would not cause the
fish any distress.

3. In a planted tank things get complicated. Increasing CO2 causes the
plants to produce more O2, and this extra O2 will probably more than
compensate for the O2 lost due to #2. The end result will be a tank that
contains more O2 and more CO2 in solution and, in accordance with #1, less
nitrogen.

Peter G. Aitken


Controller/Doser/Evaporate

by Jared Weinberger <jweinberger/knology.net>
Date: Sat, 27 May 2000

D. Grim posted

<<I can control the 
pH in my tanks to within .4 (.2 above or below the controller set point). I 
don't like the idea of injecting gas into the tank when it is not necessary, 
and the solenoid/controller does this well. I also like being able to see 
what the pH of the tank is at a glance. Probe calibration is necessary, and 
equipment can fail, but I enjoy fiddlin' with the stuff. Obviously non 
solenoid/controller methods work well for people.>>

As many folks, we leave our commercial CO2 on 24 hr -- the pH swing is
about 0.4-0.5 just as it was when we had our solenoid on with the lights.
See also George Booth's CO2 post in the last issure of The Aquatic Gardener. 

My money is going instead to a dosing pump: I've had great results recently
upping the water-column fertilization (several fold) and dividing the
amount up into 3-4 doses during the day. I'm hoping that 24-hr
fertilization will be better. I don't really know when the plants feed --
day or night? I'll test 0.2 ppm iron in the morning (Hatch kit) and then it
drops to 0.1 several hours later -- won't a steadier concentration be
better? Ditto for potassium. I have quite a few things that I add, so I've
decided to mix up a week's supply with distilled water to 3.5 liters and
dose 0.5 liters per day, thereby also replacing some evaporate. (Greg Morin
at SeaChem said that they advise refrigerating Flourish because mold can
grow once it's open, but keeping a few days' supply out shouldn't be a
problem.)

Jared
___________________________________________________________
Jared Weinberger                    jweinberger@knology.net  
______    http://www.knology.net/~jweinberger/     ________


Aquatic Plants Digest V4 #222

by "II, Thomas Barr" <tcbiii/earthlink.net>
Date: Sun, 16 Apr 2000

Thomas,

    If what you mean by 'reverse photoperiod' is to have lights on for algae
>scrubbers at night while the main tank is lighted during the day, I think I 
>know the
>answer.  It has nothing to do with nitrogen uptake. 

>    However, I suspect that it would not have much application to planted tanks.
>Most freshwater plants are used to pH and alkalinity swings.
>
>Diana Walstad

Thanks. This what I suspected..........
The reef folks trying to apply their ideas directly to planted tanks which
aren't applicable often. 

And this makes sense from natural lake and reefs also. There are those big
daily PH swings in ponds and almost none if any on reefs. But rivers and
streams don't have much PH swing though............crypts come to mind
here.........
but many FW plants are very adaptable so they can handle both well. 

Nitrogen uptake seems to be of little importance on this Reverse photo
period notion. So as was suggested that plants suppress their N uptake
during "day" to spend their energy on sugar production would it not be
preferable to add the Macro element N, say in the form KNO3, to the water
column at night then? With soil/substrate rich tanks..........as you well
know know...........the plants always have access to these nutrients so they
can use them when they need they them. Comments?  Would this help a planted
tank by adding KNO3 at night? Perhaps some trace elements as well?
Regards, 
Tom Barr 



 


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