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Re: Aquatic Plants Digest V3 #448

Hello Frank,

There is no inverse relationship between CO2 and O2 in an open aquarium
system. At *equilibrium*, the concentration of each dissolved gas is
independent of each other and depends only on the partial pressure of
each gas in the atmosphere above.

But -- and there is always a "but" -- during the photo-period in a
densely planted aquarium consumption of CO2 is high, hence one does
supplement it. At night, however, the plants stop CO2 metabolism and, if
CO2 is continually added, there will be an increase of CO2
concentration. That is well known. In most cases it is harmless.

The fish, obviously, produce CO2 all the time and get rid of it through
diffusion in the gills between blood and water. As in any diffusion
process, the *rate* of the diffusion is directly proportional to the CO2
concentration in the blood and inversely proportional to the CO2
concentration in the water. Thus, as the concentration of CO2 in water
approaches that in blood, the rate of diffusion slows down and, finally,
the exchange stops. That's the danger, because the CO2 concentration in
blood raises, leading to acidosis.

I am not certain what effect acidosis has in fish, as opposed to
mammals, but it can not be good. 

Can it lead to death? I don't know -- probably. Turning off CO2 *at
night* can not hurt the plants, because overall, at night, they produce
CO2 and it might help the fish breath easier, as a result of greater CO2
concentration differential between blood and water. So, why not turn it
off, aside from the bother?


George S  

> This is only a counter example if O2 and CO2 levels are inversely
> proportional.  I noticed another message a couple digests ago from 		> somebody who turns off his CO2 over night after major pruning,  			> apparently based on that same idea.
> I seem to recall reading (in TOA perhaps?) that there is no such 			> inverse relationship.
> Perhaps somebody who is more of a chemist than I am (not hard!) can > comment on that...
> (BTW: Finally got some pictures of my tanks up on my web page)
> Frank.