CO2 injectors: is turning off the air pump safe for my fish?

Hello everyone,

I know this topic has been discussed before, but I'm very new at this and
would like to know what I can do and not do with my aquarium. After all,
advice that's good for one particular tank may not be as good for another.

I recently got one of those Nutrafin Natural Plant System CO2 injectors.
After it started working at full capacity (providing me with long hours of
entertainment with those little climbing CO2 bubbles), I decided to get a
second one. My aquarium is 40 gallons and each canister is supposed to be
good for up to 20 gallons. I assume even one canister could help my plants,
but I figure I'm better off not doing things half-assed, else I might not
see any results at all. Both injectors seem to be working just fine, though
it's still too early to see any results.

Now, my problem is that I'm a little worried about my air pump and stone.
Since I don't want to lose the CO2 I inject into the water, I'd like to turn
the thing off, ideally for good. However, I don't know if my fish will get
enough oxygen that way, especially during the night. There's also the pH
issue to consider. It's fine if it's under 7.0, even a good thing, but I
don't want it to go too low (like close to 6.0).

As I said, I have a 40-gallon aquarium with a Fluval 304 filter. It has a
large surface area (36" x 18") and is very heavily planted. However, it also
contains a full load of fish. About 45 or so. Almost all are small fish
(many neon tetras, eight rasboras, a few barbs and a few other small fish),
with a couple of pearl gouramis. I don't think it's really excessive, but I
admit my aquarium is rather densely populated, and thus the fish must use up
a lot of oxygen.

So anyway, I'm wondering if the fish would still get enough oxygen if I
turned off my air pump and got rid of all those annoying bubbles. On the one
hand, my filter causes some surface turbulence (though nothing heavy), and I
know that a heavy load of healthy, CO2-fed plants will produce a good
quantity of oxygen for my fish. On the other hand, I do have many fish , and
turning off the oxygen at the same time I start injecting CO2 seems a bit
radical. Then again, maybe my tank contains way more oxygen than my fish
will ever need and can afford to lose some. Besides, with the air bubbles on
I'm more or less wasting the CO2 I'm injecting.

Any advice would be welcome. Thank you in advance.

Francois