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Just took my first quiz today. Got only one answer wrong in 1 min. The question was :

Air stones reduces CO2 levels : True or False

Hum... To my absolute dismay, the quiz answer is True. I answered False. Here is why.

First of all, CO2 has a better solubility in water than O2. On the other hand, CO2 has a great attraction for air. Since air contains CO2 and that you are injecting it via an air stone at the bottom on the tank, aren't you just diluting more CO2 ?

Certain people will argue that the surface agitation created with the flow of air breaking the surface will suffice to reduce CO2 levels. Not sure. Surface agitation is the best way to get rid of CO2 but if it involves injecting CO2 to start with, I wonder the total result of that equation. Less CO2 or just bubbles that flows up.

Anyway, it was fun to take the quiz. Any comments or theory I might be missing ? Feel free to comment.

Sponge

Hi,
Air stones will increase the O2 in a tank and will reduce
the CO2 in a tank.
If you leave the surface calm with no agitation, CO2
will build from the normal 5mg/l saturation to slightly
higher as the fish respire, and bacteria break down the
organic chemicals. Surface tension will keep much of the
gasses trapped within the water.
Adding airstones provides circulation, agitation, and
aeration, to the tank, breaks the surface tension, and
allows the gases free exchange with the atmosphere.

Generally speaking, in a planted tank where one is
injecting CO2 (especially DIY CO2)one tries to limit
any surface agitation so that CO2 levels are higher.
If using a bottled system, you can merely turn up the
injection rate to overcome any losses by agitation.

In some systems, because with lights out, the plants
switch from giving off O2 to consuming O2, folks will
sometimes turn off the CO2 at lights out, and turning
on airstones during the night period so that the CO2
does not build up to critical levels for their fish.

Hope this helps...
Frank

I love run on sentences

Last edited by FRANK at 20-Dec-2005 09:50

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

Frank,

Thank you for your answer. Makes sense but I would like to better understand the modecular attraction between water and gases. Any web site you might know that are reliable chemistry-wise ? I have a degree in chemistry by the way so I don't mind the tech stuff at all.

In the mean time, I'll make a search of my own but so far and from the answer you gave me, it seems I've been the victim of misinformation !

Sponge
PS : I love long sentences too !

There is a larger proportion of co2 to o2 in water, as a result of it's higher solubility constant. Likewise, organic respiration results in elevated co2. In a tank with insufficient DO, airstones increase the oxygen within the tank by allowing more air to come into contact with the water. Indeed, it is a dillution of the co2, however, proportionally, it does reduce the levels of co2 in the tank. Likewise, the surface agitation releases the co2, which has built up a higher partial pressure in the water solution, and thus diffuses faster into the air. In this manner, generally speaking, powerheads are considered superior to airstones.

Last edited by Cup_of_Lifenoodles at 20-Dec-2005 16:16

Cup,

That is more in the lines of what I read / heard. I solely rely on surface aggitation for bringing O2 to my fish and getting the CO2 out. Well... aside from the planted tanks I have, of course. But even then, during the night, I sleep better knowing that my filter outlet is at the top and so is my UV sterilizer oulet, making quite a stir at the top thought leaving my fish alone and calm for a good night of sleep.

Cheers buddy !

Sponge