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Re: CO2 Questions



Sam wrote:

> If plants take in CO2 during the day and take in O2 during the night
> wouldn't it be wise to have barely moving water during the day so that CO2
> doesn't bubble out and powerheads with air injectors turned on during the
> night to infuse O2 into the water?

Two issues that I see in your question:
1) will it be beneficial to have water barely moving during the day so
that CO2 doesn't bubble out

No.   You want to keep surface turbulence down, which will reduce
CO2 loss.  But you still need good water movement.  This is
important because a plant can only take in CO2 from the water right
around the leaf.   With barely moving water, the plants will deplete
the CO2 from the water immediately surrounding the plant.  With good
water movement, there is always CO2 enriched water flowing around the
plants.

2) Do you need to infuse O2 into the water at night?

No, not in a typically CO2 injected and well planted tank.  Plants
do not use a measurable about of O2, certainly not as much as they
product during the day.   In a CO2 injected and well planted tank,
the O2 level will be pushed extremely high during the day, and in
most cases, the O2 level will always be at or above the O2 level in
a non-planted tank using normal aeration.

> After reading quite a bit of info on CO2 I'm under the impression that it
> causes a PH swing (which I've seen) which is bad and high levels of CO2 can
> cause internal problems with fish. 

Again, several points:

1) CO2 injection will lower the pH.  But it does not need to cause pH
swings.    In my well lit and CO2 injected tank, my pH changes by
just .2 from day to night.  That is with the CO2 running 24 hours
a day.  My tank has pretty good water surface movement, and definitely
causes some CO2 loss, but I'm still able to achieve the CO2 level
I desire (15-20ppm).   The cost of the wasted CO2 is very small, so
I'm happy, and so are the plants and fish.

2) As mentioned above, in order to get CO2 around the leaves at
a rate high enough to improve plant growth, the water must contain
more CO2 than the plants actually need.   In practice, if you don't
see a pH change, then you are not supplying the plants with any
extra CO2.

3) A pH swing isn't necessarily bad.  There is no evidence I've seen
that a pH swing from day to night of even 1 full point is "bad".
The "pH shock" that kills fish is very likely related to other
factors besides the pH.   If you move a fish from one tank to
another and the tank's pH values are far apart, then it's very
likely that other water parameters are different.

> Is there any way to buffer the PH effects of CO2? For instance I was as 7.2
> and now I'm at 6.6. 6.6 is fine but when my CO2 runs out I don't want a
> quick swing to 7.2. Gradual would be more preferred.

The rate of change from 6.6 up to 7.2 will be determined mainly by the
amount of surface turbulence.  If your setup causes rapid CO2 loss, then
the pH will rise quickly.

In my case, I run a pH of about 6.8, and without CO2 it goes up to
around 7.6 or so.   When my CO2 tank needs to be refilled, I open
the needle valve wide, and allow the CO2 reactor to fill completely
with CO2.    Then shut off the CO2 line to the reactor, disconnect
the CO2 tank, and take it for a refill.   I've had the tank disconnected
for 12 hours with no rise in pH, because there was still enough extra
CO2 in the reactor to maintain the CO2 level.


> Has anyone ever built a system to limit the amount of CO2 coming from a DYI
> setup so that you can adjust your bubble rate without blowing up the yeast
> bottle?

Two options.

An easy one is to send the CO2 into a basic airline gang
valve.   One of the hoses coming from the gang valve goes to your CO2
reactor, the other one should just be submerged at the same basic depth
as the first on, but situated so that the CO2 bubbles will just rise to
the surface and be lost.  Adjust the valve so that you get the desired
amount to your reactor, and the rest is just released.   If you don't put
the second hose under the same depth of water, then it will be very
difficult to adjust, as all the CO2 will just flow thru the second line
which has the lower pressure pushing back on it.

Another option is to simply use an airstone to create some surface
turbulence, releasing CO2.  If your CO2 level is a little too high,
then increase the turbulence.  If your CO2 is a little too low,
decrease the turbulence.