

[APD] Re: CO2 off at night

John T. Fitch wrote:

> I do not understand this explanation.  It seems to me to be the opposite
> of my understanding of what happens. When the lights go off, the plants
> give off CO2 and the pH drops. If the CO2 is left on continuously, the pH
> will drop even further. When the lights come back on, the plants start
> using CO2 and the pH rises ... Is this not correct?

Yes, that is not correct. 

When the lights go off, the plants stop using CO2 and the pH goes up as CO2 
diffuses out of the tank.

Plants always respire CO2, whether they are photosynthesizing or not. When they 
are photosynthesizing, they also generate O2. 

I'm not sure if the plants respire more CO2 during the day than night or not. 
Do they sleep and slow down their internal processes?  I don't know. But they 
don't respire more CO2 at night causing the pH to drop. 

The plants themselves don't generate enough CO2 to be useful for our purposes. 
Wouldn't it be wonderful if they did? Sort of like perpetual motion. In a non-
CO2 injected tank, typical CO2 levels are about 3-5 mg/l due to fish, plants 
and bacteria respiration. With the CO2 off at night, CO2 will diffuse out of 
the tank and get to that level by the morning (see my "CO2 Loss in a Trickle 
Filter" article on my website to see how quickly CO2 is lost based on various 
factors other than the trickle filter itself).  

A further point is that most of the CO2 "usage" in the tank is diffusion to the 
atmosphere, especially at higher concentrations. I can't venture a guess on 
exact ratios, but I would conjecture that _most_ of the CO2 we inject does not 
get used by the plants.  This implies that when you turn on the CO2 in the 
morning, a lot of incoming CO2 is lost right away and it takes longer than you 
think to get to useful concentrations.

And, of course, the high tech wonder of a pH controller gets around all this 
complication by keeping the CO2 level pretty constant no matter what the 
conditions. Not without it's own minor annoyances, of course. 

 The website for Aquatic Gardeners by Aquatic Gardeners
   http://aquaticconcepts.thekrib.com/  (mirror)


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