So recently I ordered new airstones from AeroGarden as the one in my oldest unit was in need or retirement. The latest cleaning that was done on my existing airstone (as shown on AeroGarden's web site) just wasn't cutting it. When they arrived I was surprised they were not the same green color I was used to. They were this grey color as pictured below.
Well come to find out, AeroGarden had to find a new manufacturer for their little lava stones (or pumice or airstone or bubble maker or, oh you get the point). After you install one of these new ones you will definitely find that these do not produce nearly the same amount of bubbles. I would say it produces about a third as many.
The important thing in all this is not to worry, so long as it is diffusing the air and producing bubbles you should see no deterioration in your plants growth. The water is still be oxygenated and that is all that is important.
NOTE: If you find it producing barely any at all though, I have found that you can soak it in hot water for a couple hours which opens it up more.