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Re:O2 at night

> I read that it would be an improvement for the plants to supply O2 at night,

No, not really or in any practical sense.

For the critters if you overpopulate the tank, have poor plants growth(hence
low O2 produced by the plants), reduce current down too much etc you might
need to add aeration to overcome _the imbalance_ OR you could simply remove
the imbalance which would be the BEST thing in the long run anyhow for both
you, the wallet, the critters and the plants.

> which could be done with a timer and air pump.  I can't turn down the CO2 at
> night as its a DIY system and would cause too much pH fluctuation.

You ate a double myth burger:-)

First,  DIY CO2 is _EASY_ to turn on/off.
There are at least 2 simple ways to do it.

The best method is to bubble the CO2 into a powerhead's intake. Plug he
powerhead into the lighting timer. CO2 comes on only when you need it.
The bubbles do NOT get dissolved when the pump is off and simply bubble up
and escape to the air above.
A powerhead cost close to that of a air pump and is quieter. It uses the
same timer as the lights(no need to have a separate timer), it's a very
effective CO2 diffuser, it uses electric only when you need it. It can be
used on a gas tank system should you upgrade later.

Second, some folks have used solenoids, but in reverse.
When the solenoid valve closes, there is a "T" with the solenoid attached to
one of the ends. This causes the gas to go the other way, in this case into
the tank. When the valve opens, the gas is dissipated into the air. It takes
about 15 minutes for the gas to re pressurize and start bubbling into the
tank so this is not any good for Controller type systems but is fine for
timer set ups. Solenoids run about 40-70$ generally. Not really a good
option IMO but I used one since I have a few laying around.

pH fluctuation:
I have turn off CO2 at night since I started using CO2 gas. Since trying to
mimic nature is often popular, most weed choked lakes have pH fluxes of 2
units or more, some even 4 whole units each day. By contrast, springs vary
hardly at all, maybe 0.2 pH units depending on where the measurement is
taken. Further down the spring run where the plants live, the pH changes
become increasingly large along a gradient from the source of the
spring(springs are rich in CO2 generally). Basically, it's part of nature
where there are high plant/algae densities.

Adding CO2 is not about maintaining a controlled pH, it's about supplying
enough CO2 for the plants when and only when the lights are on. That's why
we add CO2, pH is a parameter to measure the CO2 with the buffer, the
KH(HCO3). Folks seem to think pH control is what it is about, it is not,
it's about getting a good CO2 level in the tank when the plants need to CO2.
ph is simply a measurement.

Beyond adding the CO2 only when the plants need it, the method does NOT
matter in any practical applications I've seen to date. But reducing
possible over dosing at night(which allows for lots of CO2 to be add during
the day easier), other equipment concerns/electrical, saving CO2 gas if you
use gas tanks and solenoids, etc etc, there is no benefit to either method.
Some folks like to say the pH stability matters at night, I say it doesn't
one bit. 

The rise in pH at night as excess CO2 is released is slow. It does no harm
even to the proverbial wimpy fish, wild Discus/Altum angels. As a tank
matures, the pH differences in the night/day in such CO2 off at night tanks
declines and the substrate tends to buffer the flux.

The notion that there is a huge pH swing or it is somehow bad for
plants/fish by turning CO2 off at night is plain rubbish and a myth.

Amano seems to do quite well as do I with all the fish plants we grow. We
both turn it off at night. Just look at Amano's sick fish and plants:-)
Page after page. Now if he is having no issues...........nor am
I.....where's the proof that the pH matters except during the day?

Show me the proof. 

>  Is there 
> any big advantage in supplying O2 at night?


Try growing the plants well, that's what it's all about. You are trying to
get the plants to produce lots of O2 during the day, the excess O2 produced
should be more than enough to make it through the night for even decent
sized fish and critter loads.
If not, work on the growing the plants better, or reducing an overpopulated
tank, cleaning the tank more often etc.

IME, DIY requires a fair amount of work to keep good CO2 levels. It is
difficult to over dose the CO2 with DIY, it is easy to reduce the current
down too much in effort to SAVE CO2. Folks often go too far and have this
happen. But that causes problems and gasping fish at the surface in the
morning if you don't have enough circulation and O2 plant production.
_Some_ surface turnover is good.
Also adding CO2 does not drive out O2. These act independently. Example you
can have high excess CO2 and O2 levels as is typically the case right before
the lights go out in a well run plant tank.

Adding an airstone at night will remove the extra O2 as well as the extra
CO2. The plants worked hard all day for that O2, so make sure not to remove
it until it drops down below 100% saturation levels(which only occurs for a
4-6 hours out of 24hrs at night and very early AM in my tanks and not much
below 100%). But the amount of CO2 lost(say 800%) will be far greater than
the O2 lost(say 50%) from aeration.
> Thanks, Sharon

Tom Barr