As you say, CO2 in the water will make your plants give off oxygen.
However,
when the lights go out the plants go into reverse and start taking in
oxygen
which is when you can get oxygen depletion in the water. Therefore what I
do
is to run an Airtone for 30 minutes or so every couple of hours through
the
night (starting at about 4 am) - just put it on a timer

I'm not sure I can get a timer that can switch on and off several times in
the same day (or night), at least for a reasonable price (I've no idea what
it costs), but I'll definitely look into setting up a timer system to make
the airstone do its job during the night. I already have a timer for the
lights, it's just a matter of getting another one for the airstone. It would
be convenient to have a more sophisticated one that can handle several
connections with various schedules.

I have a single Hagen CO2 on a 50 USG tank & it makes a big difference -
any
extra CO2 makes a difference. You don't really need two but since you have
them you may as well use 'em.

It's good to know that even a single unit can improve plants. If I find out
having two causes me trouble I'll know that using only one isn't pointless.

By the way, I got a very good deal on the units. They only cost me $25 each
(Canadian). That's part of the reason why I bought two.

SUggest you stagger the refilling & do one every 10 days or so
- that way you will have a fairly constant level of CO2.

Interesting. I might try that.

As far as the Ph is concerned it will only crash down towards 6 if your Kh
erodes to nil so just make sure you have 4 or 5 dKh & you will be fine -
your Ph will stay well above 7.

Since my neon tetras usually represent 30%-50% of the fish in my aquarium,
and since all of my othe fish are reasonably tolerant of moderately acid
water, I prefer to go below pH 7 whenever possible. The CO2 canisters are
doing a great job of it, without going overboard. Since posting my original
message I've noticed that it's been around 6.6 during the day, without
adding anything to the water. I'd also be fine with a little lower if it
happens. So it seems pH won't be a problem after all, as long as I turn on
the airstone during the night to avoid excesses.

Thank you very much for your advice. I really appreciate it. I feel a little
more confident about using the units now.

Francois