[mgeorges]

Questions:

Should I just turn down the CO2, at the expense of the plants? I'd hate to do that as I have a minor algae problem, and I think the CO2 level is actually a bit low, anyway, given the Drop Checker color.
Should I mess with the powerhead again to increase surface agitation? If so, will that cause the CO2 to decrease, at the expense of increased O2?
Should I strive for a general increase in surface agitation?

Thanks in advance for any useful info on this matter.

I would not turn down CO2, just up surface agitation. I caution against aiming Eheim's spraybar too much towards the surface...I've had a tank empty overnight because the water level dropped and it just shot water onto the floor. Not a good deal. You could aim your power head so that it creates turbulence at the waters surface. Anything that breaks the surface tension is good. As far as CO2 loss at the expense of increased O2, I wouldn't worry about it. Surface agitation is more important. You can up your CO2 a tiny bit if necessary, this is better than asphyxiating fish.

To add on to Jeff's points, CO2 should be getting shut off 1 hour before lights out.

Also, don't overdose Prime. I've killed half a tank of fish doing this, it binds O2 after it runs out of ammoia and chlorine.