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Fish gasping: Excess CO2? Or to little O2?

I have been following the discussion of gasping fish with interest.  In
my tank, when the lights go out the CO2 level goes up and the pH falls. 
If the pH falls too low (i.e. the CO2 level gets too high -- over about
20 ppm based on pH/KH chart), I see the fish really pumping their gills
in the morning, or even rising to the surface.

I always assumed that this meant that the CO2 level was so high that it
was putting a "back pressure" on the passive diffusion of CO2 out of the
gills, so the fish were gasping not for more O2, but to rid themselves
of CO2.

Does anyone know for sure what's actually happening here?  Is it too
much CO2, not enough O2, or maybe a little of both?