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Hey all! Im new to this great site and im hoping to meet many like minded people here! Im sure i will!

Back to the topic, i was taught in Biology that at night, plants respire using oxygen rather than using CO2 as they do to photosynthesise.

I was wondering if it would make any difference to the plants if the CO2 in a tank would be turned off at night to allow them to respire and also to stop the water becoming increasingly acidic due to the increased amounts of CO2 in the tank?

Many thanks!

Hi,
Welcome to Fish Profiles!
CO2 at night is a much debated topic.
First, an assumption or two... That you are using
a bottled system with a regulator, and second, that your
tank is not filled so full of fish that it resembles the
"feeder tank" at your LFS just after receiving a fresh
supply of fish.

Some folks turn off the CO2 at night as a matter of
conservation. When the plants switch from the consumption
of CO2 to the release of it, they argue that it makes no
sense to keep injecting it. By shutting it off they can
stretch the length of time it takes to empty a bottle.

Another point is that they feel that they would
be increasing the CO2 saturation while decreasing
the amount of O2 available for the fish.

In a properly functioning system, the fish will be in no
danger. The circulation of the water by the filter will
allow the exchange of gasses at the surface, and allow
excess CO2 to leave the tank while taking in O2.

Others feel that it makes no sense to add CO2 during the
day to bring the saturation to 30mg/l only to allow it to
escape overnight and have to redo the cycle again the next
day. Better to leave it run and not mess with it.

As far as expense is concerned, a refill of my 5 pound
bottle costs me 9$ and some change (tax) every 5-6 months
at the injection rate of 2 bubbles/second.

Some folks use a controller and a shutoff valve so that by
monitoring the pH and setting the controller to maintain
a specific pH, they hold the CO2 saturation constant day or
night.

Some will turn off the CO2 and switch on an air pump to
increase the circulation and surface agitation during the
night.

Personally, I leave mine running 24/7. I don't have a
controller, and by maintaining my KH at a specific reading,
and injecting CO2 at a set rate, I hold my pH at a specific
reading and maintain my CO2 saturation at 30mg/l. If I
were to turn off the gas by adjusting the regulator, it
would take all of the next day to reset it for a constant
saturation. To keep repeating that cycle is foolish.

I guess what I'm trying to say, is that its up to you.
Some do and some don't. It should be the fish that make
that determination. If you approach the tank the first
thing in the morning with the CO2 running 24/7 and you
see them gasping at the surface, then you should consider
either turning off the CO2 with the lights, or turning
on an air pump/air stone combination with the lights going
out.

In the International Aquarium Fish magazine for October
2007, page 84, Scott Hieber answers a similiar question
about airstones in a "slow growing aquatic garden."
This would be a tank with slow growing, low light plants.

Frank

-->>> The Confidence of Amateurs, is the Envy of Professionals <<<--

Whoa, awesome reply FRANk! Thanks a lot! I have no fish yet, so i guess i can experiment with leaving the CO2 on at night! Thanks again!

Sort of along the same lines, if I am dosing with excel, can I oxygenate at night with an airstone or would that have a negative effect on the regular c02 uptake??

Jay.

Hi,
If you aerate your tank it will bring the tank CO2 reading
to normal ambient levels which is around 5mg/l saturation.
If your tank has higher than normal CO2 saturation it would
bring the level down. If you have an abnormally low
CO2 level, the air stone would bring the CO2 saturation up
to the 5mg/l saturation.

However, you are using Excel and that has nothing to do
with the CO2 saturation of the tank water. Excel adds
carbon to the tank water in a form that the plants
can use, not gas, and it won't be driven off by agitation.
Just like adding Flourish Iron will increase the iron level
in the tank water Excel will increase the carbon in the
water, not Carbon Dioxide in the tank water.

As far as the carbon uptake at night, the plants don't
utilize carbon during the night hours when they
cease taking in CO2 and releasing O2.

The only time the use of an air stone in a planted tank
is discouraged is when one is using DIY CO2 injection.
The supply of CO2 is limited by the yeast/sugar mix and
its temperature. It is a finite amount and the mix and
its age and temperature determine the quantity of CO2 that
is produced as the fermentation takes place. When the mix
is fresh, there is lots of CO2 produced. When the mix ages
the CO2 is reduced, eventually, to nothing. When using
DIY you want to use every bit of the CO2 produced that you
can. To keep the CO2 in the water and limit the exchange
that takes place between the bubbles and the tank water
with the atmosphere, you want to not rile the surface and
instead, maintain the surface tension of the water.
This means no air stones, and the filter returns are under
the surface so the surface integrity is maintained.
With a bottled system you can monitor the CO2 saturation,
and, if necessary, merely turn up the regulator needle
valve to produce the desired saturation and compensate for
any losses due to surface agitation.
Frank

-->>> The Confidence of Amateurs, is the Envy of Professionals <<<--