What is an 'air stone'?

What is an 'air stone'?

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An air stone is a little, porous, literally like a little stone that attaches to air line tubing, which is then attached to an air pump. A vibrating pump will force a lot of air through that air tubing, and then through the air stone and you'll get anywhere from fine to coarse bubbles that will come out of that air stone and again, oxygenate the water mostly as they break on the surface