Hi @Andy PierceI don't think why the airstone works is known, but the effectiveness doesn't seem in dispute. One theory I've read is that when the bubbles break they fling the biofilm in every which direction, some of which winds up getting flung outside of the tank.
Many thanks for your reply. I was beginning to think on similar lines to what you have said above. But I wasn't sure if my imagination was just having a field day!