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Re: CO2, gasping, etc

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 (booth at frii_com)
    http://www.frii.com/~booth/AquaticConcepts/