An airstone is a device attached to the end of a hose from an airpump for releasing many small bubbles of air instead of fewer bigger ones. Some of them are actually made of coarse natural sandstone, others are made of pressed and glued grains of coarse sand, or synthetics, plastics, ceramics, even wood. All of them are for aeration or moving water up a lift tube. Some people think the stone actually forces air into the water but that's not so to any appreciable extent, the real air exchange takes place at the water surface, a moving water surface exchanges more gas and the aeration is to move the surface and circulate the water. The airstone makes a soft fizzing sound that can barely be heard instead of the louder Bloop Bloop of big bubbles. The air stream moves the surface and circulates the water in a constant gentle stream instead of causing weird waves and periodic more powerfull currents that bother the fish. In a lift tube, to or from a filter for instance, the finer bubbles actually move more water than big bubbles. There are also stones for decorative purposes, a narrow one as long as the tank placed against the back wall makes a of wall of risning bubbles that looks sort of like a water fall. The stone can become clogged over time reducing air flow and straining the pump. Small ones are cheap and can be replaced. Larger ones can be boiled for a while which does help. Yum. yum airstone soup.