Air stones break the stream of air produced by your pump into thousands of tiny micro-bubbles, which help aerate your tank. The more airstones you add, how large they are, and how porous they are (less porous materials like wood produce more resistance) will help you decide how powerful your air pump needs to be. If you have a protein skimmer, check the kind of air stone it needs. Make sure that you clean or replace stones when they slow down; they clog over time.
