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Re: Oxygenation vs. CO2 loss

On Fri, 5 Jan 2001, John T. Fitch wrote:

> On Thursday, 4 Jan. 2001,  "Roger S. Miller" <rgrmill at rt66_com> had a
> suggestion for someone having difficulty getting enough CO2 into the water.
> He said: "Also turn off or readjust anything else you're running that might
> aerate the water or cause surface turbulence."
> This brings up a question I have about how to direct the discharge from the
> output of my Fluval filter system: (1) above the water level,

Noisy, messy and unecessary.

> (2) at the water level,

Will allow CO2 to escape and limit your CO2 concentrations.

> or (3) below the water level.

Best for a planted tank.

> I have a 30-gallon tank with an
> M3 CO2 system, which, through a so-called Mixer J, injects the CO2 into the
> output from the filter and heater modules before discharging the return flow
> into the tank.  Obviously, I want to provide whatever oxygen is required for
> the fish, without wasting CO2.  I would describe my mix as a medium number
> of plants and about 30 inches of slim-bodied fish.
> I have read, somewhere, that (1) above, i.e. actively splashing water into
> the tank causing bubbles below the surface is unnecessary and that just
> rippling the surface, (2) above, will provide all the oxygenation the fish
> need. But Roger Miller's advice appeared to suggest even that was
> unnecessary and that (3) above would be best.  Of course, I couldn't tell
> from the context of his answer whether he was considering a mix of fish and
> plants or just a planted tank.

Your need for aeration depends on the fish load and a few other factors.
My sense of it is that if you need to aerate the water to support the
fish, then you probably have too many fish for a CO2-injected planted

Plants aerate the water pretty effectively when they have sufficiently
bright light and the necessary nutrients (CO2 in particular).  My "show"
tank seems to approach O2 saturation about 2 hours after the lights
come on and go well into supersaturation a little after noon.  During
the day the fish get all the O2 than can use.

There is a potential for low oxygen after the lights go out.  When the
plants stop photosynthesizing they become net users of O2.  I suspect that
in my tanks the plants satisfy most of their oxygen needs from the gas
that builds up inside the plants when the lights are on, so they don't
cause much of an oxygen deficit at night.  A few people have reported
problems with low oxygen in the morning before the lights come on.  One
solution for that problem (if it arises) is to keep an airstone in the
tank.  Run the airstone with a pump on a timer set to keep the pump
running while the lights are off.

Roger Miller