air in tank

Well, the main purpose of an airstone is to oxygenate the water. This doesn't happen directly from the air pumped in, though; instead, the bubbles cause the surface to ripple, which improves surface area and water turnover so that more O2 can enter the water from the air. You can create the same effect with flowing water, such as the outflow from your hang-on-back filter.
 
If your overflow is creating sufficient surface agitation; if the water level is right up to the overflow you may not be getting much agitation.

In any case, an airstone certainly can't hurt.
 
I don't use airstones in my tank, I have plenty agitation from my HOB's output. However, I don't want too much agitation because I pump CO2 in my tank and with too much agitation, the CO2 gasses off into the air around you.
 
Any body have some unbiased documentation that a bubble traveling thru water can't exchange gases...
 
I'm quite sure it can, it's just that the transit time is brief. Some insects use air bubbles as a 'physical gill', allowing them to remain underwater indefinitely. In this case the air bubble is actually absorbing O2 from the water.
 
Molecular gas transfer does not take very long...maybe micro seconds.
 
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