The air stone fits in the base of the biOrb underneath the filter cartridge. It breaks up the flow of air into the aquarium, splitting it into a mass of fine bubble.
Ensure steady stream of air bubbles into biOrb
If the bubbles have reduced in your biOrb you may need to replace the air stone. Over time the tiny pores in the air stone can become blocked by dirt and lime scale preventing air from getting through properly.
It also reduces noise from the air pump.
Remove old airstone & replace with new one.