As matter of fact, I was just looking this up yesterday and found this
article... The following is an excerpt and below that is the entire link:

At night, on the other hand, the plants are actually using oxygen (and not
CO2) If there are not too many fish in the aquarium, then the oxygen
produced by the plants during the day will tide everyone over until the next
morning. However, if you notice that your fish are gasping at the surface in
the mornings, they are obviously running out of oxygen. To remedy this
problem, you can simply turn on an air stone when the lights go out. This
will keep up the oxygen level and remove excess CO2. I have the aquarium
lights and an air pump on two separate timers; when one turns on, the other
one turns off. It would also be fairly easy to rig up a solenoid valve for
the CO2 supply and have it turn the CO2 on and off with the same timer that
is regulating the lights.

Here's the link:

http://faq.thekrib.com/plant-co2.html

cheers,
Boris

"Dave M. Picklyk" wrote in message
news:TI8kb.118953$6C4.48157@pd7tw1no...
I just thought I'd toss this around the newsgroup and see what kind of
opinions I get: What does everyone think about adding more aeration at
night
when the lights are off and plants have reversed their gas roles. I have
seen a few of my fish dwelling towards the water surface at night. I was
thinking about the possibility of adding a timed airstone for dark
periods.
I am concerned about fluctuating PH, but more about oxygenation for my
larger bodied fish.

Thanx for any help you can give!

Dave.

www.picklyk.com/aquascape