Diffusion Stones, or 'Air Stones', are commonly used to aerate wort before fermentation, which helps ensure a healthy start to the fermentation process. Diffusion Stones can be attached to compressed oxygen tanks or air pumps (such as those used with aquariums). This "Stone" is made of stainless steel, so it's durable, easy to clean, and won't crumble in your wort after a few uses like other materials! Less air pressure is required to force gas through the the 2 µm stone compared to stones with smaller pores, making the 2 µm stone ideal for use with small air pumps. But beware that many lightly powered aquarium pumps may still produce insufficient pressure to force air through the stone and into the wort.
The Diffusion Stone is also suitable for force carbonation of beer inside a keg. But the larger bubbles produced by the 2 micron pore size will limit the absorption rate of gas into the beer.