"Poe Lim" wrote

What's your level of CO2 (ppm)?

I'm afraid I don't have what it takes to measure it. I've no idea how much
CO2 I have. Probably too much. But that's because the plants aren't using
enough of it due to the lighting conditions. I'm using 2 Hagen injectors,
which are supposed to be ok for just about 40 gallons (like my aquarium),
each being good for up to 20 gallons. And one of them is running out of CO2.
If my plants were healthier I don't think it would be too much.

If I were you, with low light, drop the level of CO2.

The more I think about it, the more I think I should stop injecting CO2
until I have more more light. As it is, the plants don't get enough to make
good use of the CO2. The CO2 accumulates, not much used by the plants, and
the plants aren't healthy enough to produce a good amount of oxygen. It's
not surprising that the neons don't like the gas situation.

FWIW, CO2 and O2 can exist independently of each other in
water, but sounds as if the level you have is too high for the light you
have.

I agree. I'll figure out a way to add more light, then I can go back to
injecting CO2. I can't know for sure, but I suspect it will work wonders. A
lot more light and an extra dose of CO2 that the plants can actually use
efficiently. Along with the weekly dose of iron-based fertilizer I add to
the water, the plants should be in vegetable heaven. I'm looking forward to
trying it.

In my tank, with CO2, you can actually see the leave bubble O2, which
means the O2 level is supersaturated, and the fish don't have any
problems.

Strangely, I used to observe that phenomenon in my tank a long time ago, say
two years ago, during the first year. It was a steady stream of tiny little
bubbles coming out of some of the plants and floating to the surface. Unless
it was just a gas leak coming out of damaged leaves, I don't know. Anyway,
my plants were doing great back then, even though the light conditions were
the same. Not sure why they've been having more trouble ever since. It seems
to me that, if anything, I've been taking better care of them over the last
two years. I don't mean the CO2, by the way, as that's a very recent
addition.

Also be aware that there may also be a layer of CO2 on the surface of the
water due to CO2 being heavier than air, which may reduce O2 exchange.

Interesting. I've never thought of that. Makes sense.

In any case, while I understand that there's probably too much CO2 in my
water compared to oyxgen, I still don't know why the neons are hanging in an
area that's may be a bit richer in CO2. And even if it isn't, what is it
about the injectors that they like so much? One would expect the opposite.

Thank you very much for your advice.

Francois