   An airstone is a piece of aquarium equipment, traditionally a piece of limewood or porous stone, whose purpose is to gradually diffuse oxygen into the tank, eliminating the noise and large bubbles of conventional air filtration systems, and providing other benefits to the health of the fish. Airstones are sold in a very wide variety of shapes, sizes, and levels of coarseness--from extremely rough, producing larger (though still typically unnoticeable) bubbles and letting in more oxygen--to very fine, producing minuscule bubbles.
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