Odd neon tetra behavior with CO2

Hello everyone,

This question is about fish, but since it has to do with CO2 injection,
something planted aquarium people are rather familiar with, I figured this
newsgroup was relevant enough. My apologies if you feel this is off-topic.

A few weeks ago I started adding CO2 to my water, and my neon tetra have
been behaving in a way I don't understand. First, I noticed that if I leave
the airpump off for too long the neons have trouble getting enough oxygen.
Or at least that's what it looks like. The breathe really quickly and even
go to the surface when the airpump is off for too long. I'll eventually set
up a timer system that will turn on the air bubbles at night, but for now
I'm doing it by hand. The neons don't seem capable of being comfortable a
full day without air bubbles, which I find annoying (especially since the
other fish look just fine), but it's not their fault, and if they need more
oxygen, then so be it. My CO2 system will simply ne allowed fewer hours per
day of maximum efficiency, that's all. No big deal.

Of course, I understand the lack of oxygen issue. What I don't understand is
why the neons tend to hang right next to the CO2 injection point after a few
hours without air bubbles. The CO2 injector is on the right side of the
aquarium, in the back, behind rocks and a big piece of wood. Normally the
neons don't have any reason to hang there. At least they never did before I
started using CO2. Now, if the neons have oxygen problems when the airpump
is off, why would they hang right next to the CO2 source? It seems to me
that, if anything, the CO2 is more concentrated in that area, and there may
be less oxygen. It doesn't make sense to me.

At one point I thought it might be a pH thing. Neons tend to prefer acidic
water, and more CO2 means a lower pH. However, my overall water pH tends to
be about 6.6-6.8 (due to the CO2 injector), which should be fine for the
neons. And when I took a sample in the CO2 corner and tested it, the result
was the same as for the rest of the water. I don't see why they would prefer
the corner for pH reasons.

So, if the aquarium's pH is just fine for the neons, and if the neons aren't
especially fond of long hours of CO2 injection with no air bubbles, why
would they hang in the back next to the source of their problem? If they
really are more comfortable there, good for them, but it doesn't make sense
to me. I also miss seeing them swimming around in the front, where they used
to hang before I started adding CO2.

One thing that should help in the near futre is an increase in the amount of
light my plants get. I've always known I don't have enough light, but I only
recently realized just how bad the situation is. At the moment, I only have
about 0.6 watts/gallon (a single 25 watt tube for a 40 gallon tank). I
understand 2 or even 3 wpg would be closer to what I need, so it seems I
desperately need to add a few lamps, which I intend to in the next few
weeks. I'm hoping my plants will do a lot better, using more of the injected
CO2 and producing more oxygen. If it happens, the neons should fare much
better. I hope they stop hanging in the back corner then.

Any explanation for my neon tetras' behavior would be appreciated. Thank
you.

Francois