I found this with a quick google search. This is from an aqueon rep:
"The bubbles have everything to do with the way the glass expands when filled. As the tank is filled the largest panels begin to bow out. This is normal and entirely to be expected. As it does bow out the inside edge of the seam becomes stretched causing small voids to form in the silicone. This is normal and has no effect on the structural integrity of the aquarium. It will also generally appear from the center up to the top because the top does not have the silicone seam of the bottom glass holding it together which reduces the movement (bowing is highest in the center of the panel and decreased at the bottom). You can see this pretty dramatically if you've ever filled a plastic or rubber trash can with water."
"Certain tanks are going to show these bubbles more than others. Thinner glass, larger volumes, taller tanks are all going to increase the amount of bow. It doesn't take a lot to have the bubbles start to show up (perhaps a few thousands of an inch?)."
