Wood airstones are supposed to be one of the bests. But as I found out after buying a pack, it depends on how powerful the pump is. Even my biggest pumps and pond air pumps don't make micro bubble clouds like a protein skimmer need. It requires a deep water air pump which are over $100 easy. D'oh!
Petsmart has a cheap granite looking stone in diff sizes that produces microbubbles like a wood one but last longer. Wood only last few months apparently. Still need monster pump