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CO2 off at night?

> From: George Slusarczuk <yurko at warwick_net>

> 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?

The bother is big to me: I have to go purchase a solenoid and wire it in
with the lights.

But the big reason is that for many systems, most of the CO2 we inject is
normally diffused back to the air instead of used by the plants.  By
turning it off, the pH will creep up several points to its "non-injected"
eqilibrium with the air.  For instance from pH 6.8 to 7.5... And then the
next morning, back down it goes.  Compare with leaving it on, in which the
pH will lower a little in response to the plants being out of the system,
but will stay mostly the same because of the high diffusion rate to the

My suggestion is: try it!  Leave the CO2 on 24 hours for several days, and
measure the pH right before the lights go on, several hours after, and
right when they go off.  Maybe in the dead of night too.  If there's not
much fluctuation, you've got no worry about fish poisoning. Then try it
the other way... turn it off at night, repeat test.  Depending on how
closed your system is, you may favor one approach or the other. (See, for
instance, George Booth's loss rate experiment at
http://www.cco.caltech.edu/~aquaria/Krib/Plants/CO2/co2-loss.html which
shows the majority of the CO2 being cleared within 8 hours for even the
most closed aquarium system, less than four hours for most others.)

   - Erik

Erik D. Olson					         amazingly, at home
eriko at wrq_com