Hi all,
Yes, definitely turn the CO2 off, and leave the air pump on 24/7 during the black out.
The plants will be respiring (oxygen (O2) in and CO2 out) all the time, what normally happens is that as the light brightens after lights on or at dawn the plant will start to photosynthesise and, assuming the light is intense enough, it will reach compensation point where the O2 use and CO2 production from respiration matchs the CO2 use and O2 production. Any increase in photosynthesis after this level will lead to O2 production and CO2 depletion. Adding CO2 stops this depletion happening and causes more photosynthesis, often leading to "pearling", where the water is 100% saturated with oxygen and it out gases as bubbles.
No light = no photosynthesis = no CO2 uptake and no O2 production.
Light drives both photosynthesis and nutrient up-take by the plant, so no light means not only no oxygen production but much less uptake of NO3 etc. the plants are then adding to the bioload, with the nitrogen cycle in the biofilter having both less O2 and more NH3 to deal with. This is why the tank may become oxygen depleted while the lights are off and you need additional filtration & aeration.
Have a look here: <
http://plecoplanet.com/?page_id=829>
cheers Darrel
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