Air stones

[Swampman]

Hi. I had a discussion with my brother. He uses an air stone in his tanks during the day and shuts off after lights go off. He said it puts CO2 in the tank. I said oxygen. What is the correct answer. Thanks

[rlederer]

It does both. Basically an air stone disturbs the surface water and causes gas exchange between the air and water. Gas exchange of different gases will occur depending on the concentration of the gas in the air and water. Both carbon dioxide and oxygen will enter the water but there is way more oxygen in the air than carbon dioxide so way more of it will enter the water, day or night.

[Swampman]

Thanks so much.

[paparam]

Composition of room air:
Nitrogen (78%),
Oxygen (21%),
Argon (1%),
Carbon di-oxide (0.03%)
Other trace amounts of multiple gases, water vapor

[Caliban07]

It will depend on lots of factors but room air composition and the concentration of oxygen to
Carbon dioxide will mean little when you consider the solubility of both gases. You can only really achieve around 8.3mg/l of oxygen in water. That would be a fully saturated water column and it would need to be pure H2o at 24 degrees at sea level. It’s not very soluble yet we know we can inject 30mg/l of carbon dioxide and beyond from a compressed source.

The airstone is also important, the finer the bubbles the more oxygen will be introduced. The disturbing of the surface works off concentration gradients. That is what dictates how much oxygen comes in and how much goes out as well as other gases like co2. Eventually an equilibrium will hold steady.

If the concentration of co2 increases (which is true in tanks with a heavy bioload) and the gradient between the concentration of co2 inside the fish and what is in the water column isn’t sharp enough, the fish can struggle to expel the co2 and gasp at the surface. But this isn’t lack of o2. This is too much co2. But this is why I run the airstone or sponge filter in my case 24/7. To provide a safety net.

But this is why plants are so desirable, they take up that co2 and produce 100% o2, as opposed to the air which only has 21%. You can fully saturate the water column this way which is seen by the oxygen bubbles released by the plant.

Salinity, air pressure, temperature, airstone type, surface area to volume ratio of aquarium, bioload or biological oxygen demand, chemical oxygen demand, clarity of surface (no biolfilm etc), planted or non planted, healthy plants. All these things effect how much oxygen the water will hold. Fish can also be oxygen depleted before they even show signs of distress.