Question on new tank

Thanks to the information I've gathered from this list, I have a new heavily
planted 75-gallon tank that is thriving. But a problem has cropped up that I
would like some advice on.

I started the tank three weeks ago last Wednesday. I'm using an Eheim
canister filter, gravel with Dupla laterite for the substrate, daily doses of
Dupla drops, 160 watts of Triton fluorescents and DIY CO2.To get the tank
started I put in four rosy barbs from my old tank along with some Cycle.
Things have gone well and along the way I added three SAEs, a silver dollar
from the old tank (I know they like plants, but as long as I keep romaine
lettuce in the tank, the silver dollars don't bother the plants too much),
and three panda corys and three red-green corys. Plant growth has been
phenomenal, and ammonia, nitrite and nitrate have been non-existent (by the
way, I added more Cycle each time I added more fish). Although Chicago water
normally has a pH of about 8 out of the tap, the CO2 has taken it down to
6.5, which the fish are agreeable with. The kH is 3.

This past Wednesday (three weeks from the startup of the tank, I returned
from being out of town for nearly a week. Everything looked very good and all
water tests were good (again, no ammonia, nitrite or nitrate). I transferred
the remaining silver dollar, two rasboras and a gold severum from my old
tank, and they settled in nicely. The following morning, Thursday, several
fish were hanging around the top of the tank, suggesting oxygen problems.
After the lights were on a few hours, everyone was acting normal. Again, all
water tests were OK.

This morning, Friday, two barbs were dead and fish again were at the top of
the tank. Again, after a few hours of lights on, everyone was acting OK. My
assumption is that oxygen levels are falling too low when the lights are out
(they are on 12 hours a day). Talking with the fish store I usually frequent,
they advised putting an airstone hooked to an airpump into the tank and
discontinuing use of the CO2 when the lights are out. I've put in the
airstone but frankly I'm hesitant to use the CO2 only during lighting hours
as I'm afraid the pH will be bouncing up and down.

So, my questions are:
(1) Given that the plants are growing like crazy and the number of fish in
the tank is minimal, why aren't the plants producing enough oxygen on their
own to keep things going smoothly overnight?
(2)  Am I going to have to keep an airstone going permanently in the tank?
(3)  And, am I right in thinking it's a poor idea to be shutting down the CO2
every 12 hours?

Any answers will be appreciated, and thanks for all of the information I've
gained by reading your posts in the past.