Bubbles coming up from the gravel?

My vote is for bacteria. A single algae wafer does'nt contain much air, and loses a good bit of it on the way down. Anerobic bacteria on the other hand produces hydrogen sulfide (deadly stuff) and may have been breaking down your wafer under the substrate.

Finger sift the gravel in that spot for a few days, just to make sure you don't get a anerobic "bloom".
 
This happens all the time in my 20 long, when I gravel vac, numerous bubbles come up just as you are describing. All the fish are fine, and no smells or anything. Ive often been curious as well as to what this is.

Blue
 
Please dont be caught up in this whole deadly gas idea. The amount of it needed to kill your fish would almost never occur unless you payed little to no attention to your tank.

If you want to know some more about it, you should check out some articles at reefcentral.
 
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