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Re: CO2 reactors queries and hypoxia

>> All you need is a small gap in the cover glass to fix that.  Is there any
>>  reason other than water evaporation to have a cover?  If not, you don't
>> have to seal it to contain the majority of the evaporation.
>There is an inconsistancy here, on the one hand you say you only need a 
>little gap to stop asphyxiation and on the other you say that you do not 
>need to seal the container to contain the majority of the water loss. To 
>my mind if both are gases escaping from the aquarium they will do so at 
>nearly the same rate and the rate at which oxygen enters will be similar 
>to the rate of loss.

Okay, let's do this thought experiment then:

I have a tank with a cover that has a 1" gap.  Using pH and KH, I know that
I'm injecting more CO2 than the atmospheric pressure of CO2.  Knowing that
my tank pH and KH have been fairly stable for months, I know that the plants 
and substrate aren't absorbing the excess CO2 (or they would have pulled up 
the pH), therefore the excess has been leaving the water at the surface.

If the CO2 has been accumulating, I would have had an increasingly higher CO2
pressure over the water and the partial pressure increase would keep more CO2
dissolved in the water, lowering the pH. Since the pH has NOT been decreasing,
I can conclude that CO2 has not been accumulating.

Similarly, if the CO2 had been displacing O2, the partial pressure of O2 would
have dropped and forced lower dissolved O2 levels, which would have killed my
fish if my plants couldn't keep up.  Because I know the plants can't keep up
with my CO2 production, I know they can't produce O2 at the same rate.  Thus
if my fish aren't dead by now, I can conclude that the O2 is not displaced.

As a last measure, when I remove the glass cover, I can observe nearly a 4X 
increase in the evaporation of water, therefore I can conclude that with the
cover I've reduced the evaporation rate, which was my primary goal.

One last note: CO2 does not displace O2 in the water.  You can find the stable
CO2 and O2 levels in the water from their partial pressures in the atmosphere.
When you add CO2 through a reactor, you've increased the steady-state partial
pressure of CO2, bringing the dissolved CO2 levels up independently of the O2
levels.  Water can dissolve an immense amount of particles...there are an awful
lot of water molecules in a tank for hydrogen bonding.

Carlos Munoz
cmunoz at crystal_cirrus.com