Airstone

An airstone is an article made of soft stone, porous wood, or another material that allows air to be blown through it to produce bubbles in water. The air is blown through the stone by a tube placed inside the stone and attached to an electric pump. 
An airstone can be used in an aquarium (a tank or container for keeping fish) or a tank of water where plants are growing in a hydroponics system.The airstone is used to blow bubbles into water to increase the amount of oxygen in the water. The stone may have very small holes to make fine bubbles or it may have bigger holes to make larger bubbles.